Wow, I nominate this as most vague question of the year. Does it crash? Does nothing happen? Show us what you did to create the button.
It should be:
Position: 7
Icon: Protoss Probe
Req. Type: Can create unit/building
Act. Type: Create Unit
Req. Var: 64
Act. Var: 64
String/Req. String: Whatever
and requirements for Probes should be:
Current unit must be [next requirement]
Terran Marine
Current unit must not be constructing
If you want a stat bar, you could copy the Misc./Disp. functions from the Barracks (Misc is Unk. 3 and Disp. is Unk. 2) and see if it shows the green bar. I did so with a Psi Emitter, and the green bar only updated when you did something, but it worked with a Bunker. (Which happens to also have the "Produces Units(?)" unknown checked, along with the Barracks.
But, when unchecked, the Barracks still updated the progress bar, and I didn't test the Bunker.)
Anyways, if that crashes, there's probably some iscript work involved.