I fucked up what do i do

I moved to California and I had to sell my truck for cash upfront and I did not know you needed a transfer and release title thing Bob mind you I’m 18 and when I asked for them to help me fill it out because they had already bought the truck and left with it they blocked me. What do I do? I’m getting notices from the DMV and I don’t know how to tell them. I can’t get a transfer when they have my title and everything already any advice