A table can have at most only one column of type SERIAL and one column of type SERIAL8/BIGSERIAL. You are attempting to add a second column of type SERIAL or SERIAL8/BIGSERIAL, or you are attempting to create a table with more than one column of either type. If you intended to have a foreign key (that is, a column that refers to a SERIAL or SERIAL8/BIGSERIAL column in a different table), the data type of the column in this table should be INTEGER or INT8 or BIGINT.