You click Save, Install or Delete and WordPress shows a plain page: "Are you sure you want to do this? Please try again." Nothing was changed. Every admin form carries a one-time security code, and WordPress refuses the action when that code is missing or too old. The question is why the code went missing: the page sat open too long, the file you sent was bigger than the server accepts, a cache served an old copy of the page, or a plugin's own form is broken.
Reload the page and try once more. The code lasts about a day. If the tab was open since yesterday, or you were logged out and back in on another tab, a fresh page fixes it and you are done.
Were you uploading a theme, plugin or big file? If the server's upload limit is smaller than the zip, PHP throws the whole form away, the security code with it, and WordPress asks "Are you sure". Tools, Site Health, Info, "Media Handling" shows the "Max size of an uploaded file". A zip bigger than that is your answer.
Does it happen on every save, for everyone? Then something is serving a stored copy of the admin page with an old code. Look for a cache plugin or host cache that caches logged-in pages, or a CDN rule that caches /wp-admin/.
Only on one plugin's screen? Deactivate that plugin (or, if you cannot tell which, all plugins) and try again. A plugin updated without its form being updated is a common cause; switching to a default theme for one test rules out the theme the same way.
Started after adding a firewall or security plugin? Some strip or block form fields they do not recognise. Its log will show the blocked request.
The fixes, by case
Page open too long: reload and redo the change. If it keeps happening on long edits, save drafts more often.
File bigger than the limit: ask the host to raise upload_max_filesize and post_max_size (64M covers almost every theme), or set them in the host panel's PHP settings. Or skip the limit: unzip the theme on your computer and upload the folder to wp-content/themes with the file manager or SFTP, then activate it in Appearance, Themes.
Cached admin pages: in the cache plugin, turn off caching for logged-in users and exclude /wp-admin/ and wp-login.php; purge the cache. Do the same in a CDN's page rules.
A plugin or theme: update it; if the error stays, tell its developer the exact screen and button, and use the previous version or an alternative until they fix it.
A firewall rule: allow the blocked request in the security plugin's log, or ask the host which rule fired.
Test: redo the action once on a freshly loaded page and confirm the change is actually there afterwards.
What you see, what it means, what it needs
What you see
What it means
Usual fix
Once, after leaving the page open for hours
The security code expired
Reload and try again
Every time you upload a theme or plugin zip
The zip is bigger than the server's upload limit
Raise the limit, or upload the unzipped folder by SFTP
Every save, every user, since a cache was set up
Admin pages are being cached
Exclude logged-in users and /wp-admin/ from the cache
Only on one plugin's settings page
That plugin's form check is broken
Update or replace the plugin, report it
Since a security plugin or firewall went on
A rule is stripping the form
Allow the request, or ask the host
What not to do
Do not install a plugin that "disables nonces" or turns the check off. The check is what stops another site from tricking your logged-in browser into deleting or changing things.
Do not keep clicking the same button. Nothing is saved on this page, and repeated uploads of a too-big file will never get through.
Do not re-install WordPress. The core is fine; the form arrived without its code.