My first guess would be that you are running a noncontemporary version of osTicket or are not running PHP 7.4 like you said you are.
There is no way for us to know what direction to attempt to troubleshoot this without more information.
It's sort of like calling a mechanic and saying "Hey my car isn't working. It throws an error."
What is the make and model of the car? I don't know.
What year is the car? I dont know.
What engine does it have in it? I don't know.
What webserver software are you running?
What version of PHP are you running?
Do you have more than one version of PHP installed on the server?
What version of osTicket are you running?