Need some electrical help with my ‘68 Plymouth Valiant 100

Hey gang, I am new here and was told to get some help from this forum about my issue.

Well the major issue is everything flickers/trembles and it sounds like it is too. At night, the headlights/brakelights, ceiling dome light, dash light all flicker, I notice that the ammeter needle will bob slightly too, it intensifies when driving and it will dip below the halfway mark when I’m in drive holding the brake at a light.

I went to O’reilly’s to have them check out the battery. Apparently, the battery wasn’t bad so to speak, but said it needed a charge. Then I turned the car on so we could check the alternator and there was no reading listed when it was checked.

I’m trying to understand what could be the exact issue. Is it the alternator itself? The regulator? A fuse or ground issue? Like I said, I’m definitely complete novice, so apologize in advance for my lack of knowledge.