it's the regulator. the voltage come from the flied connection on the regulator to the alternator. If it does not then your alternator will never charge.

As a very temporary test you can run a jumper from the battery positive to the field connection on the alternator and measure the alternator output, it will charge if the alternator is good. What you have just done is bypass the regulator altogether and told the alternator to put out now.

Just run this test long enough to verify alternator output.