ERROR: cannot execute CREATE TABLE in a read-only transaction

Normally the most plausible reasons for this kind of error are :

• trying create statements on a read-only replica (the entire instance is read-only).

• <username> has default_transaction_read_only set to ON

• the database has default_transaction_read_only set to ON

The script mentioned has in its first lines:

CREATE DATABASE exercises;
\c exercises
CREATE SCHEMA cd;

and you report that the error happens with CREATE SCHEMA at line 6, not before.

That means that the CREATE DATABASE does work, when run by <username>.
And it wouldn’t work if any of the reasons above was directly applicable.

One possibility that would technically explain this would be that default_transaction_read_only would be ON in the postgresql.conf file, and set to OFF for the database postgres, the one that the invocation of psql connects to, through an ALTER DATABASE statement that supersedes the configuration file.

That would be why CREATE DATABASE works, but then as soon as it connects to a different database with \c, the default_transaction_read_only setting of the session would flip to ON.

But of course that would be a pretty weird and unusual configuration.