I can't access the SM from my country. But I have had this same trouble with some Sony VHS decks, and the JVC machines may use a similar mechanism.

There are one or more plastic sliders (long thin parts) that move to the right and the left under the cassette compartment. The mechanism changes modes by sliding those parts so the brakes are engaged/disengaged the idlers contact the reel tables, and so on.

A lot of Sony machines behave exactly as you say, because a plastic lug has broken on one of the sliders. (To this lug a small spring is engaged, that goes to another lug on a different slider).

If you can't find a spare slider, sometimes a tiny hole can be made on the slider where the original lug was, and a small screw put on it, to serve as a makeshift lug.

But it is a jeweller's job.

I see you are from Cuba, and while I'm not very aware of the details reguarding Internet services there I would try using a VPN...

That or using a TORR browser would probably bypass the firewalls.

I ran across this video on YouTube..
https://www.youtube.com/watch?v=mvjjGN4VAPw

He repairs a JVC VCR that I think is from around the same era as yours. He mentions audio problems and shows the bad caps. I'm not sure if your VCR has the same audio PCB, but it might.