Not giving me the option to build derelict cars?

I pieced together the first derelict car (the mustang) but now that I put the second car and the third car together, there isn't an option to build it. When I go to the garage, the mechanic makes a comment about how we should put it together, but thats it. I've been through every menu and there's nothing. I don't know what I am missing. Is there a button i'm not pressing?

I've tried swapping to another character, but that doesn't work either. I've tried restarting and it's still gone. What am I doing wrong?