He played there when Warnock initially signed him, lasted for a handful of games before he pushed him into midfield.
This is a direct quote from him on playing RB for Scotland
"It’s not my favourite position. I like to attack and I was a bit exposed. It’s not the best for me but I’d do whatever is asked for me for the country"
Scotland probably play him there because they have no other option. When we had the same issue last season during Sanderson's drop in form Harris rather play Bacuna there. Says all you need to know