You need to first alter your TableA to add an extra column (i.e yourNewColumn) of type varchar/varchar2
to add the column try something like
ALTER TABLE TableA ADD COLUMN yourNewColumn varchar(10);
Then you can use something like below to update TableA
UPDATE TableA
SET yourNewColumn = ISNULL(TableB.countries, 'none')
FROM TableA
LEFT JOIN TableB ON TableA.id = TableB.id
The ISNULL function in PostgreSQL might be something like
SET yourNewColumn = CASE WHEN TableB.countries IS NULL THEN 'none' ELSE TableB.countries END