Problem:
While trying to export a select result, you got the following error message: Relation xxx does not exist

Possible Solution:
If you are working with Temporary Tables, be aware they will be available for this session only.
So, when exporting the result, make sure you are not allowing the client to OPEN A NEW CONNECTION.

Otherwise the new connection will not know the tables, since they only exist on the previous connection.

Stupid Issues, but it's worth to share ;)

Dbeaver
PostgreSQL
Squirrel SQL Client