There is no page on your website where it where it says PHP7.4 is required !
Even if this is true, then why version 3.8.1 works fine ? what code have you added to not to be compatible with PHP8 ? It absolutely makes no sense, because at the end of the year your code will simply be completely obsolete. this is like walking backwards !
NOW ! I suspected you are wrong about PHP version and now I can prove it. I switched back to PHP 7.4 , cleared CACHE and browser cookies. I reopened back end of the site and got into the same problem. The netire section with the Serial Key is missing so the SPB works as DEMO !
I do not know how long are we going to play this Ping-Pong gane but I am totally frustrated with this. I have Helix+SPB installed on 3 websites running Joomla-4 and there is exactly the same problem on all of them.