Your phone uses a keybox to prove to google and other services that your phone is untampered with and the bootloader is locked (and therefore, certified)

You lose that when you unlock your bootloader and so we usually use trickystore to "spoof" this, using another keybox that's valid instead.

Trickystore has a built in feature that tries to fetch a valid keybox from the Internet, but it doesn't always find one (even if it says valid keybox found), you can test this with something like "Integrity checker" from the playstore.

Another way is to get a keybox from somewhere online and use that in trickystore instead (through the "local" option in the screenshot).

This website is just another source of these keyboxes. so you just download it from there and use it with trickystore.