It is really correct that there is no tag for perfect substitues?
There is one for complements.
Can someone perhaps create it?
Yes, there was no tag before. New tags are usually created by high rep users when they fit some question. There might be some topics that nobody bothered to ask or they might get tagged with just more generic tags like microeconomics for example.
I created the tag for you by tagging your latest question with perfect-substitutes tag.