What does 'The plugin does not have a valid header' mean in WordPress?

WordPress cannot find the plugin's main PHP file, the one whose top comment contains 'Plugin Name:'. Usually the plugin sits in a folder inside another folder, or the uploaded zip is a package that holds the real plugin zip.

Will I lose plugin settings if I reinstall a plugin to fix the header error?

No. Plugin settings live in the WordPress database, not in the plugin folder, so removing and re-installing the plugin's files keeps them.

More fixes

WordPress plugin install error: "The plugin does not have a valid header"

Q: How do I fix 'The package could not be installed. No valid plugins were found'?

Open the zip on your computer. If it contains another zip, upload that inner zip instead. If it is a GitHub download, rename the folder without '-main' and re-zip it. If it contains style.css, it is a theme and installs under Appearance, Themes.

You upload a plugin zip or click Activate, and WordPress stops with "The package could not be installed. No valid plugins were found." or "The plugin does not have a valid header." The plugin is usually fine. WordPress just cannot find its main file where it expects it. Here is how to tell which case you have, and the fix for each.

What the owner sees

harborbakery.example/wp-admin/plugins.php

What the checks found

Find out which case it is (five minutes)

When did it appear? During "Upload Plugin" ("No valid plugins were found"), the zip is packed wrong or is not a plugin. On "Activate", or right after a plugin vanished from the list, the plugin's folder on the server is wrong.
Open the zip on your computer. A good plugin zip holds one folder (for example contact-widget/) and inside it a .php file whose top comment starts with Plugin Name:. If the zip holds another zip, or a folder inside a folder, that is the fault.
Is it really a plugin? A theme zip (it has style.css), a "full package" download from a marketplace (it has documentation and a licence file next to the real zip), or a source download from GitHub (named like plugin-main.zip) all fail this way.
Look on the server. In the host's File Manager or FTP, open wp-content/plugins/. The plugin's folder should contain its main .php file directly, not a second folder of the same name.

The fixes, by case

Zip inside a zip: unzip the download on your computer. Upload the inner zip that is named after the plugin, not the whole package. Marketplaces like CodeCanyon call this the "installable WordPress file only" download.
Folder inside a folder: on the server, move the inner folder's contents up one level (or delete the plugin folder and re-upload a zip that has one folder only). Then go to Plugins and activate it again.
GitHub download: rename the folder from plugin-name-main to plugin-name, zip that folder, and upload the new zip. Better: download the release zip from the plugin's Releases page, which is packed for WordPress.
It is a theme: upload it in Appearance, Themes, Add New, Upload Theme instead.
Main file renamed or broken: if you edited the plugin, the comment block at the top of the main file must still start with /* and contain Plugin Name: Something. A missing line or a file saved with a byte-order mark hides it. Re-download the plugin rather than patching it.
Plugin "deactivated because the plugin file does not exist": the folder was renamed or moved, often by an update that failed halfway. Re-install the plugin from Plugins, Add New; its settings are kept in the database.

What you see, what it means, what it needs

What you see	What it means	Usual fix
"No valid plugins were found" on upload	The zip has no plugin main file at the top level	Upload the inner zip, or re-zip one folder
"The plugin does not have a valid header" on Activate	WordPress looks for the main file in the wrong place	Fix the folder-in-folder on the server, then activate
Zip named `something-main.zip`	A GitHub source download	Rename the folder, or use the release zip
Zip contains `style.css` and screenshots	It is a theme	Install it under Appearance, Themes
"Plugin file does not exist" after an update	The plugin folder was renamed or half-replaced	Re-install the plugin; settings stay

What not to do

Do not download a "nulled" copy of a paid plugin to get one that installs. Those copies are the most common way WordPress sites get hacked.
Do not add a fake Plugin Name: line to some random file to make the error go away. WordPress will then load the wrong file.
Do not delete the plugin's rows from the database to "start clean". Re-installing the files is enough, and the settings are worth keeping.
Do not keep uploading the same zip. If it failed once, it is packed wrong, and every try can leave another half-made folder in wp-content/plugins/.

Most cases are one zip opened and one folder moved. If the plugin still will not install, that is what the box below is for. Related: plugin install or update failed, "Add New Plugin" button missing, "file type is not permitted".