how to remove offhand weapon with shield from centaur
 

Lances while use on foot do not let you use shields with them but mounted characters can use shields with lances. How do I allow centaurs and Wemics to use lance and shield with out hero lab from giving a validation error?

Is this something that Hero Lab can correct it is not an error but something
that regularly does not come up because Centaurs and Wemics are not standard character races.

I would suggest submitting a bug report, preferably with some documentation references ("rulebook X says..." or "FAQ Y says...") so that LoneWolf can research the issue properly.

Have you tried checking the mounted condition?