I'm having this issue as well and I went to look though the files. This line is already in place so is there any other options to try? The only benefit of this error is that the repeat quests count down days dont progress leaving you plenty of time to upgrade your slaves but without the option to sleep with them to get their lewdness up :(
Viewing post in Bug
Are you using any mods? Because the fix should only be present if you are using a mod or my bugfix patch.(Edit: or if you added it yourself, in which case you made a mistake or used a low quality text editor) If you aren't using my patch, I recommend getting it from Discord as stated in a post above.
If the error message does not exactly match the one in the first post, then please post it here.
You cannot have both the working fix and the error, so one of those is different.