In your copy / paste exercise, either the source cells or the target
cells are merged, which is causing your error message.

I'm trying to think of a workaound, but the only idea coming to mind
is to unmerge whatever cells are involved. As a rule I avoid merging
cells, since it's typically a cosmetic issue and not a functional
value-add. That's totally subjective, a preference of mine, but it
short-circuits problems such as this one.

Just my $0.02. I usually get change back whenever I give my 2 cents!