Is it okay to true (align) a wheel with a missing spoke?
No, you cannot true the wheel with a spoke missing - at least not as a permanent solution. I'd only do this to get home or to a bike shop if it was not a long ride.

Remember that spokes alternate coming from the drive (right) and non-drive side (left) of the hub, and a properly trued wheel balances tension from spokes from either side.

If you have a drive side spoke missing the two non drive side spokes on either side of its position will pull the rim toward the left, you would have to slacken these spokes because they are not pulling against the missing spoke. This is going to cause tension imbalances in the wheel which will stress other spokes and accelerate their failure.