Alter table add column with no default blocks
The problem is not your ALTER TABLE, but other transactions in the database.

You must have some long running transactions that hold any lock on the table (the ACCESS SHARE lock from a SELECT is sufficient to bloch the ACCESS EXCLUSIVE lock of an ALTER TABLE).

Your ALTER TABLE has to wait until this long running transactions are gone, and any later transactions that are trying to access the table have to queue behind the ALTER TABLE.

Close the long running transactions, and the statement will complete in a jiffy.