Hi!
Just to double check, was the game opened on an ongoing playthrough (started before the last update from Sunday 8-June)? If so, yeah, that's because the new update introduced a variable (for the Achievements), and broke old saves. Just click Restart Game in the Restart/New Game popup, and that'll fix it.
If your game was started after Sunday 8-June, could you send me your save file of that playthrough (either Discord/email)? Because that shouldn't be popping up.
Glad you enjoyed the game regardless!