It sounds like you have missing values in your dataset, matchcn. I don't know the twang package, but you probably need to subset your data to exclude cases where there is an NA value for the subject. There are lots of ways to do this in R, see answers in the Stack Overflow thread below:
I'm afraid I'm a little out of my depth in terms of what you're asking. If you could make a reprex, it would go a long way towards helping others help you:
A self-contained reprex (short for reproducible example) means that others can reproduce exactly what you're seeing.
There's also a nice FAQ on how to do a minimal reprex for beginners, below:
What to do if you run into clipboard problems
If you run into problems with access to your clipboard, you can specify an outfile for the reprex, and then copy and paste the contents into the forum.