Unable to grant permission on table
At a guess, your user isn't the owner of the tables and hasn't been granted rights to them WITH GRANT OPTION so it can delegate them.

The database administrator must do something like:

GRANT ALL PRIVILEGES ON ALL TABLES IN SCHEMA public TO myuser WITH GRANT OPTION


and should probably also GRANT similar rights on SEQUENCEs, on the schema its self, etc.

The WITH GRANT OPTION gives the user myuser the right to in turn GRANT the assigned rights to others.

See the manual for details.

You might also be interested in ALTER DEFAULT PRIVILEGES.