The light should go off immediatly after fixing the cap. If not, it should after the next 10 miles or so.
If not by then - open the hood.
Look closely at the "stuff" on the left splash panel, almost under the brake booster.
Find a small "TEE" in a vacuum line with a cap over it.
Remove the cap, put a longer hose on the Tee - maybe 2 or 3 feet.
Engine off - blow cigarette smoke in that hose - a big puff.
Look for where the smoke comes out - fix that! It may be close by or underneath.
That's the evap system access - it's supposed to be sealed.
If you've done any work near there, or under it, a line may have been knocked loose. Or sometimes a bush or debris gets kicked up underneath and severs a line.
IF - you can't find any leak, then follow the procedure in the factory service manual for that code.
Or - do a Google search for the procedure.
But --- I've never had to go further than the smoke test.
After fixing the leak the light should go off - at least within 10 miles.
Tip - For those of us that are afraid even one puff will get us stared smoking again.
Use a big turkey baster - suck the smoke in that, then shoot it into the Tee. Works great!
(After a few days, smell the turkey baster - YUK! How could we have enjoyed that so much?)