This idiom or expression means:

In trouble, as in:
When someone says “If she finds out, my neck will be on the chopping block,” they mean something like “I’ll be in trouble.”

Other usage:
When you put someone on the chopping block, you’re making them the scapegoat, and putting the blame on them.
If a company is laying people off, and you are on the chopping block, it could mean that you are among those being considered for termination.