Your inner SELECT query is returning more than one value - so SQL rightly complains that it can't test multiple values against a column with an equality test.
If you are trying to get a specific row which matches one ID value, then you need to refine the inner query until it will only ever return a single value.
If you are trying to return multiple rows with different ID values for the same data, then use IN instead of an equality test:
SELECT * FROM DBO.mp_racun_roba WHERE id_fakture IN (SELECT IDColumnName FROM mp_racun_lista WHERE datum=CONVERT(date,GETDATE()))