Hello, I just finished my complete playthrough of The Walking Dead Telltale series and came across no issues, except for one achievement not unlocking in the final season, I have tried everything but it seems like it's not just me because someone on my friend's list has the same issue with the same achievement. Is there a way to fix this or do I just need to accept it? Thank you.

It depends. There are several questions here:

1) Is it a game related problem or a Galaxy related problem? In Both cases you should create a support ticket and hope that GOG will be able to fix this.

2) Some Achievements are stored in the savefile and some will only unlock the moment you achieved them. So if it is the first - after GOG hiopefully fixed it - you would just need to reload your latest save for it to unlock. However in case it is the latter you would have to achieve the achievement again ... meaning you'd need to replay at least this part of the game. Knowing TTG a bit, only the latest state of the game is saved so it will be hard for you to get this done without replaying the complete game ... unfortunately TTG removed the rewind option they had in earlier games.

If you'd ask me - such cases are not high priority for GOG, especially right now ... so maybe it might be the best to just accept it right now. Creating that ticket however won't hurt - maybe you are lucky. Good luck!