You click Update on a post, save a page builder layout, upload a file, or a visitor sends your contact form, and instead of saving you get "Not Acceptable! An appropriate representation of the requested resource could not be found on this server", or just "406". WordPress did not make that page. Your host's firewall (almost always ModSecurity) read the request, matched a rule, and refused it. Nothing on the site is broken; one rule is too strict for one thing you are doing. Here is how to prove it and get it fixed.
What the owner sees
harborbakery.example/wp-admin/post.php
What the checks found
Is it the firewall? (two minutes)
Look at what you were sending. A 406 that appears for one post, one form message or one file, and never for others, is a firewall rule matching the content. Common triggers: words like select, union or exec next to code-like text, a pasted <script> or <iframe> embed, a long URL with query strings, or a file name with odd characters.
Try a small test. Save a copy of the post with the suspicious paragraph removed. If that saves, the content tripped the rule.
Check the host's log. In cPanel, Errors or ModSecurity shows the blocked request with a rule id number, the page, and the time. That id is what the host needs.
Fix it, safest first
Ask the host to whitelist that one rule for that one page. Send the time, the page (for example /wp-admin/post.php or /wp-json/) and the rule id. A good host removes that rule for that path in minutes and leaves the firewall on everywhere else.
If your panel has a ModSecurity switch (cPanel, ModSecurity, per domain), turning it off for a few minutes is a fair test: if the save works with it off, you have your proof. Turn it back on and use step 1 for the real fix.
A security plugin can do the same thing. Wordfence, Sucuri and others run their own firewalls. If the block page names the plugin, check its blocked-requests log and allowlist that action there.
Cloudflare or another proxy. If the block page carries a Cloudflare Ray ID, the rule is in Cloudflare's firewall, not the host's. Its Security, Events screen names the rule; add a skip rule for /wp-admin/.
Visitors' forms fail. If messages with links or certain words never arrive, the same rule is blocking your contact form, and you lose enquiries without knowing. Send yourself a test message with a link in it after any fix.
What you see, what it means, what it needs
What you see
What it means
Usual fix
406 on one post, others save
Content in that post matched a firewall rule
Host whitelists the rule id for the editor path
406 when saving a page builder layout
Large request full of code-like text
Rule whitelist for admin-ajax.php or /wp-json/
406 on a contact form with a link
Visitor messages blocked by the same rule
Rule whitelist for the form's path, then a test send
Block page names a security plugin
The plugin's firewall, not the host
Allowlist in that plugin's blocked log
Block page with a Cloudflare Ray ID
Cloudflare firewall rule
Skip rule for /wp-admin/
What not to do
Do not leave ModSecurity switched off for the whole site. It stops a lot of automated attacks; the fix is one rule on one path.
Do not keep rewording the post until it saves and call it fixed. Visitors' form messages may still be blocked by the same rule.
Do not reinstall WordPress or delete plugins. The 406 comes from the server in front of WordPress.