I think the problem is you need to specify NEW.invoice in all comparisons
(don't quote the 'column')and always, Always, ALWAYS end each statement
with a semicolon.
IE:
IF (NEW.invoice <= 'I-10000') THEN INSERT INTO myschema.mywork VALUES (NEW.*)
ELSE IF (NEW.invoice >= 'I-10001' AND NEW.invoice <= 'I-20000')
THEN INSERT INTO myschema.mywork VALUES (NEW.*);