My mother dies 20years ago with a 401k left and since then the spouse died, but never claim her 401k. Who does it go to.

Who was the beneficiary on the 401K was there a second one listed. It will go to any secondary beneficiary if listed. It goes to the estate if not

I'm the son, she tried to make me the sole beneficiary, but she did not get the waiver signed by her husband.

Do you know who is holding the 401k at this time? That is who you need to ask. They should tell you if you were listed or not. If his and your name were on it and he has passed then it should go to you.