I cant choose PostgreSQL

Even tho im under student offer from github, which says that its equivalent to pro, I cant access it

you need to have a credit card at least, dedicated DB runs a specific postgres container just for you so you kinda need to pay to use that

https://appwrite.io/docs/products/databases/postgresql#specifications

and also as the doc says "it's available to all paid plans" so possible that the student, since it's free, doesn't have access to that feature