a command to build something when enough resources are available
Is there a queue-able command to tell a structure to "build/research this when enough resources are available"?
No.
That is part of the very strategy that goes into multiplayer matches.
It would break the foundations of the game if you could queue build orders without having the resources.
One could simply queue up an entire list of building and units as well as movements and not even partake in actually managing their resources or using micro.