We are continuously expanding capacity, so you can try again later, however, the immediate solution is to create your instance in a different zone.
I'll recommend reviewing this
link that recommends distributing your instances across zones to increase availability. You can also review this other
link, where you will find some best practices for designing robust systems on GCE.
Getting a "zone resources exhausted" message is specific to that request and that moment in time. It is possible that the size or type of resource you requested is temporarily in use, try other available zone (if a different zone not crucial ) or try in different time.
Unfortunately, there rare occurrences, therefor why we are continuously expanding our capacity.