Can't launch session, validation error.

I can’t launch session out of no where. This error Pops Up [via Photo]. I got no errors in Message Log or Output log, everything works fine. I even tried to validate the project and no errors found and it says it was validated successfully. Any help? I also deleted every asset we added since we were in session for the last time. Even after this it does not work. Please help, we are almost finished with the project.