There is a pressure test port on the main gallery, see if you get pressure there.
I have redone an engine with this "no pressure" situation. I found that previous "clutch basket removal" repairs
which had accidentally reversed a behind the basket flanged washer had also moved the oil pump gear out far enough that it could disengage from the clutch basket gear.
This same engine was also missing the small "anti-spin" pin that makes a positive connection for the gear behind the basket that drives the oil pump, without the "anti-spin" pin it was slip city under any real world oil back pressure from the oil pump.
I have seen engines with clutch side cover gaskets that permitted all oil pressure to dump out before making it to the jug stud pathway to the head.
I saw one gasket with NO THROUGH HOLE in it for the oil pressure to make the trip to the oil filter.
I have seen bad crank rod male passage to female cover lip seals (badly torn) that did the same thing, dumped out all the pressure.
I have heard of people losing the little metering jet (located next to the speedo gear & spud) that goes to the transmission that can come out when you take the side cover off, venting way too much oil down the tranny and starving the head.
There is a whole stack of gaskets/"o"rings on the jug base, the head gasket, the top cover that must be done correctly or you cut off the vertical oil path (this assumes you didn't cut it off earlier by doing one of the things above.
Lastly, the little drilled passages up in the head cover love to get blocked off by silicone gasket maker "ooze" & loose chunks of that same substance that get loose down below.