Why do I get a PostgreSQL permission error when specifying a tablespace in the "create database" command? When I create a database in PostgreSQL without explicitly specifying a default tablespace the database is created without issue (I'm logged in as the pgsys user):

postgres=> create database rich1;
CREATE DATABASE
postgres=> \l+
                                                                            List of databases
   Name    |  Owner   | Encoding |  Collation  |    Ctype    |          Access privileges          |   Size    | Tablespace |                Description
-----------+----------+----------+-------------+-------------+-------------------------------------+-----------+------------+--------------------------------------------
 postgres  | pgsys    | UTF8     | en_US.UTF-8 | en_US.UTF-8 |                                     | 7455 kB   | pg_default | default administrative connection database
 rdsadmin  | rdsadmin | UTF8     | en_US.UTF-8 | en_US.UTF-8 | rdsadmin=CTc/rdsadmin               | No Access | pg_default |
 rich1     | pgsys    | UTF8     | en_US.UTF-8 | en_US.UTF-8 |                                     | 7233 kB   | pg_default |
 template0 | rdsadmin | UTF8     | en_US.UTF-8 | en_US.UTF-8 | =c/rdsadmin                         | 7345 kB   | pg_default | unmodifiable empty database
                                                             : rdsadmin=CTc/rdsadmin
 template1 | pgsys    | UTF8     | en_US.UTF-8 | en_US.UTF-8 | =c/pgsys                            | 7345 kB   | pg_default | default template for new databases
                                                             : pgsys=CTc/pgsys
(5 rows)

As you can see, the database is put into the pg_default tablespace, but if I specify the default tablespace in the tablespace clause (also still logged in as pgsys) I get a permission error:

postgres=> create database rich2 tablespace pg_default;
ERROR:  permission denied for tablespace pg_default

Here's the permissions for that user:

postgres=> \du pgsys
               List of roles
 Role name | Attributes  |    Member of
-----------+-------------+-----------------
 pgsys     | Create role | {rds_superuser}
           : Create DB

This is PostgreSQL error, but I should mention that this an AWS Aurora instance in case that makes a difference.
Here's the trick, at least with pgAdmin v4:
Leave the tablespace blank. It will default to "pg_default" when creating the database.