If no other fixes work, the database needs a reset. This does not delete VDI pools, but it resets the licensing service.
The Horizon Administrator console can be misleading. Use the command-line tool on the Connection Server. horizon connection server license key is invalid
: Sometimes the "invalid" status is purely a formatting error. It is easy to accidentally copy a trailing space or a hidden character from an email. Pasting the key into a plain text editor first to strip formatting can reveal these invisible gremlins. If no other fixes work, the database needs a reset
(Invoking related search suggestions.)