I have a goodman furnace, 1994 model. When you turn it on you get a constant light (normal condition) but the ignitor will not glow. After a short period I get a continuous flash. The code table says there is a "twinning" issue and to check the polarity. Don't know what this means or what to check.

To check the polarity, you need a voltmeter capable of reading 150 volts AC. Check the black in coming wire to ground. You should get +/- 120 volts AC. Then check the incoming white to ground. You should get virtually nothing. If you get voltage on white but not on black, reverse them.
My guess is the polarity is OK but you have a bad board. The twinning thing only comes into play if you have two furnaces tied together.