I assume this was a running engine, and now it’s not. A little more info on your build would help with diagnosis.
With the key in the run position check the field wires to the alt for 12v. One of them , I forget which one, green I think, should have 12v. This excites/energizes the alternator. If you don’t have 12 volts going in, the alt will not work.
The 12v field wire is powered by the voltage regulator, so check to see you got power going in & out of the VR with the key in the run position. This is likely your problem… could just be a bad ground, so try it with a separate ground, jumper wire.
If you do have 12v going into the alt and still no output when running check both ends of the output cable for continuity and good clean connections. Do you have 12v at the starter relay, or did you do an ammeter bypass and run the output wire directly to the battery? If all the connections are in order, and it cranks over, you prob have a bad alternator.