Low beam lites out bulbs good - 1991 Oldsmobile Ninety Eight Regency

In the diagram for left low beam, the top wire color is voltage, black wire is ground. Check them both.
Have you tried swapping bulb from one side to the other to see if problem follows the bulb. If it does, then of course the bulb is the problem. Don't touch bulb with bare fingers. I can't rule out a wiring problem? At the proper fuse did you check for voltage, use a test lite.

change the bulbs if they r to fase bulbs

If you give us year--make--model, I'll try to look up wiring diagram?

you need a new light bulb. there are two different bulbs for high and low beam or 2 filements in one bulb. ether way you need a new light bulb

CHECK THE BULBS FIRST! TRY REPLACING WITH A NEA ONE ON ONE SIDE, AND TURN LIGHTS ON, IF YOUR LOW BEAM LITES UP, REPLACE THE OTHER ONE! IF NOT THERE COULD BE A WIRING PROBLEM TO THE LOW BEAMS

make sure its the correc bulb--chec over the connection and look inside lite-maybe new but broken

if the high beams function when holding the passing lever, you know the bulbs and fuses are good, the bulbs do operate. the passing lever bypasses the headlight switch on the dash. replace the headlight switch, and be prepared to also replace the connector for this switch, since they had a tendency to melt.

Check the wire circuit for voltage and ground. The top wire is voltage, you can see the wire color, use a test lite, with lights on. The bottom black wire is ground for low beams, chassis ground. You can use test light to check ground, With test light touching voltage, touch ground wire terminal, if the light comes on, at least, the ground is good enough to lite the test light.

Test to see if you have voltage at the bulb connector in low beam position. I'd almost bet that if there is no voltage there for the low beams that you have a bad high/low switch.

try scrap yards.