What does 'XML declaration allowed only at the start of the document' mean on a WordPress feed?

An XML feed or sitemap must begin with <?xml as its very first characters. Something on the site printed a blank line, a space or an invisible byte-order mark first, so feed readers and Google refuse the file.

How do I fix a broken WordPress RSS feed with 'error on line 2 at column 6'?

Make sure the theme's functions.php and wp-config.php start with or blank lines at the end, re-save them as UTF-8 without BOM, and if the feed still breaks, switch to a default theme and turn plugins off one at a time to find the one that prints the line.

Why does Search Console say my WordPress sitemap could not be read?

Often the same stray blank line is printed before the sitemap's XML. Remove it from the file that prints it, purge caches, check view-source shows <?xml on line 1, and resubmit the sitemap.

More fixes

WordPress feed or sitemap error: "XML declaration allowed only at the start of the document"

Q: How do I fix a broken WordPress RSS feed with 'error on line 2 at column 6'?

Make sure the theme's functions.php and wp-config.php start with or blank lines at the end, re-save them as UTF-8 without BOM, and if the feed still breaks, switch to a default theme and turn plugins off one at a time to find the one that prints the line.

Q: Why does Search Console say my WordPress sitemap could not be read?

Often the same stray blank line is printed before the sitemap's XML. Remove it from the file that prints it, purge caches, check view-source shows <?xml on line 1, and resubmit the sitemap.

You open /feed/ or /sitemap_index.xml and the browser shows "This page contains the following errors: error on line 2 at column 6: XML declaration allowed only at the start of the document". Feed readers, Mailchimp RSS campaigns and Google Search Console then say the feed or sitemap "could not be read". An XML file must start with <?xml as its very first characters. Something on the site printed a blank line (or a space, or an invisible byte-order mark) before it. The usual source is a theme or plugin file with an empty line before <?php or after a closing ?>. The XML itself is fine; only that stray line has to go.

What the owner sees

harborbakery.example/feed/

What the checks found

Find out which case it is (five minutes)

Look at the line number. "Line 2" (or line 3, 4) means one or more blank lines came first. "Line 1, column 4" or similar usually means invisible bytes: a byte-order mark saved by a desktop editor.
See the stray line yourself. Open the feed address with view-source: in front of it (for example view-source:https://yoursite.com/feed/). If the first line is empty and <?xml sits on line 2, that is the fault.
Is it only the feed, or the sitemap too? If both break, the blank line comes from code that runs on every page: the active theme's functions.php, wp-config.php, or a plugin. If only a sitemap breaks, look first at the SEO plugin that makes it and anything that hooks into it.
What changed last? A snippet pasted into functions.php, a hand edit to wp-config.php, or a plugin installed or updated just before the feed broke is the place to start.

The fixes, by case

Take a copy first. Download the files you are about to touch with the host's File Manager or FTP, so every change can be undone.
Check the theme's functions.php: in the host's File Manager, open wp-content/themes/your-theme/functions.php. The very first characters must be <?php, with no blank line or space before them. At the end, delete a closing ?> and any empty lines after it; the file does not need one. Do the same for wp-config.php in the site's main folder.
Re-save without a byte-order mark: if the top looks clean but the error stays, the file was saved with a "BOM". Re-save it as "UTF-8 without BOM" (Notepad++: Encoding, UTF-8; VS Code: click "UTF-8 with BOM" in the status bar, Save with Encoding, UTF-8) and upload it again.
Find a plugin that prints the line: switch to a default theme (Twenty Twenty-Four) in Appearance, Themes and reload the feed. If it still breaks, turn plugins off one at a time (or rename a plugin's folder in wp-content/plugins/ by adding -off) and reload after each. The one that fixes the feed is the culprit: update it, or replace it.
Clear caches, then re-check: purge the caching plugin and any CDN, open view-source: on the feed again, and confirm <?xml is now on line 1. In Search Console, resubmit the sitemap; in a feed reader, refresh the feed.

What you see, what it means, what it needs

What you see	What it means	Usual fix
"error on line 2 at column 6"	One blank line before `<?xml`	Remove the empty line before `<?php` or after `?>` in the file that prints it
"error on line 1" with odd characters, or nothing visible	A byte-order mark at the start of a PHP file	Re-save the file as UTF-8 without BOM
Feed and sitemap both broken	Code that runs on every page prints the line	Check `functions.php` and `wp-config.php`, then plugins one by one
Only the sitemap broken	The SEO plugin or something hooked into it	Update the SEO plugin, then test the other plugins against it
Search Console: "Sitemap could not be read"	Google hit the same stray line	Fix the line, then resubmit the sitemap

What not to do

Do not install a "fix my feed" plugin that buffers and trims the output. It hides the stray line instead of removing it, and the next one appears somewhere else.
Do not delete and reinstall the SEO plugin or turn off sitemaps. The sitemap is usually fine; the blank line comes from another file.
Do not edit core WordPress files such as feed-rss2.php. They are not the cause, and an update overwrites them anyway.
Do not edit functions.php with the built-in Theme File Editor if you cannot also reach the host's File Manager: one wrong character there can take the whole site down.

Most cases are one blank line or one re-save in one file. If you cannot find which file prints it, that is what the box below is for. Related: "headers already sent" warning, site not showing on Google and PHP warnings on the site.