I picked up an AT1 that has been sitting for 30 years or so. I'm assuming it is neutral because rear wheel moves without pulling in clutch. It will not go into any gear. When you move the foot shifter it seems like it's not moving either UP OR DOWN into any gear.
I have not done or checked anything YET...
What might be the possible causes and things to check???

Think you'd better do a little more investigating then come back. Keen to advise but--when you move the lever to the max up & down do you feel anything like its trying to turn the selector drum or just goes to a solid stop. Chain on. Front sprocket come off. Trans oil tell a story if you drain it. Any pics or video please.

Sorta feels like it's trying to turn selector drum. Chain is on. Have not drained or checked oil. Sorry I know it's not much to go on. Just wanted to get an idea of what I MAY be up against once I start digging into things.

I got a 75 DT100 for cheap 10 years ago.  The trans would not shift.  It sat for 20 or so years before I got it.  Turned out that condensation had formed in the trans and the shift drum and shift fork sliders had surface rust that froze up the trans.  I had to split the cases and replace those parts.

Hopefully your problem will be outside the main cases, like the shift mechanism under the clutch cover.

Good luck!

Unusual, so if you hold the lever hard 'up' & roll forward there's no feeling of wanting to click in!!. Not sure where to start realy. Could pull the left cover & see if the chain has come off prior to you having it & if its bunched around the front sprocket it can wipe off the end of the selector drum above sprocket under a three screw little cover. Maybe an unlikely thing but gotta start somewhere. Does the clutch feel like its lifting & normal. Thinking has the clutch center nut come off. I'll leave it there for now.

I got a 75 DT100 for cheap 10 years ago.  The trans would not shift.  It sat for 20 or so years before I got it.  Turned out that condensation had formed in the trans and the shift drum and shift fork sliders had surface rust that froze up the trans.  I had to split the cases and replace those parts.

Hopefully your problem will be outside the main cases, like the shift mechanism under the clutch cover.

Good luck!

Yes could very well be as 'jayb' says in #4. Could remove the three screw cover above the sprocket & try to turn the drum with pliers but if it does turn you'll need to revolve the sprocket shaft a little back & forth to get the gears to go through. BUT something i hadn't thought of 'is' --is the neutral ball & spring somehow stuck up. Can't imagine it but worth a try. Its the 17mm hex bolt behind the neutral light switch on the case above the sprocket--sort of.
www.partzilla.com/catalog/yamaha/motorcycle/1970/at1b/shifter-1