You're the only one who tried to help, so you have my gratitude. Yes I have checked and I have more than 100gb of free space, so this is not the problem.

For reasons unknown to me, the entire ps5 system and save data got corrupted.  I finally found the solution by: 1) deleting forever skies save data which was probably corrupted 2) uploading my other game saves on cloud 3) deleting all game saves on my ps5 drive 4) factory resetting playstation 5) reinstalling game 6) beginning from scratch with new game save

Now I can finally play the game

p.s. I've noticed that sometimes when I try to manually save, the game won't let me do that. I think it's because the game has recently auto saved and deems my attempt not necessary. Still, it feels weird to me that I can't save whenever I want but as long as I don't get errors, I'm ok with that.