cannot get simple PostgreSQL insert to work

Use 'auto dealer' instead. PostgreSQL interprets " as being quotes for identifiers, ' as being quotes for strings.

Also:

• If this is a new project, just don't use mixed case tables; it is a source of frustration later. Instead of being able to use any case in your SQL statements, you must both quote the identifier name and get the case correct.

• There is no need to specify id/DEFAULT, you're asking it to do what it would have done already. I haven't met a DBMS that requires you to include columnName/DEFAULT if you want it to put the default value in the column, so I don't think this extra KV pair is going to make what is happening clearer to anyone reading your code later.

INSERT INTO "imageTagBusinessMainCategory"
("businessMainCategory")
VALUES
('auto dealer')

EDIT: Added double-quotes around the column name

Postgres, Oracle etc expect the column name to be in quotes if they have mixed case. So either create a convention of all small or all caps for your table columns or use quotes as David Faber suggested

INSERT INTO "imageTagBusinessMainCategory"
("businessMainCategory")
VALUES
('auto dealer')