BethINI is failing to write Fallout4Custom.ini inside the C:\Users\KJRCr\OneDrive\Documents\My Games\Fallout 4\BethINI Cache\<generated-name-backup-directory> folder. Please verify that the INI Path provided for Fallout 4 is the correct location (C:\Users\KJRCr\OneDrive\Documents\My Games\Fallout 4\). Also, check if BethINI successfully created the BethINI Cache folder. If there is a log file in there, you can upload it to Pastebin and link here.
This bit of information has led me to being able to fix the issue: the BethINI.ini file I kept merely glancing over then, turned out to be the problem - as you have rightly concluded - as it was incorrectly pathing to an old directory prior to when I moved my FO4's installation folder. I have now since rectified the error, and have successfully used your tool.
Thank you once again for your assistance and for your efforts in the modding community.
I wish you all the best.