Recently, Apple released the iTunes 12.6 and introduced the new function "Rent once, watch anywhere" to its users. According to the release notes, this new feature enables Apple users to play iTunes movie rentals across all devices with iOS 10.3 or TVOS 10.2. That is the good news, but the bad one is that the update of iTunes causes many problems to some DRM removal programs before they are updated to be compatible with iTunes 12.6.
Such as Tuneskit M4V Converter for Mac, TunesKit Apple Music Converter for mac, Tuneskit Audiobook Converter for Mac, etc.
Since iTunes was updated to 12.6, many users have encountered the same issue and sent email to firstname.lastname@example.org for seeking a help. Actually, TunesKit has released the updated Windows version of M4V Converter, Apple music Converter, AudioBook Converter to apply to iTunes 12.6. The updated Mac version will be released soon.
Tutorial: Downgrade iTunes 12.6 to 12.5
Step 1: Uninstall iTunes 12.6 from Mac
1. Quit iTunes 12.6 on your Mac.
2. Go to Applications > iTunes app file (/Applications/iTunes.app)> Get Info.
3. Click on the padlock icon (on the bottom right side of the window)> Enter your admin password.
4. Then expand the Sharing & Permissions >Adjust the privileges for “everyone” to Read & Write.
5. Close the Get Info window > Delete iTunes application file.
Step 2: Reinstall iTunes 12.5 on Your Mac
You can download the previous version of iTunes from iPhonefirmware.com and then install it on your Mac.
Step 3: Rebuild iTunes library
To make iTunes library work for the older version, in this step we need to rebuild the iTunes library.
Go to your home directory > Music > iTunes and find the file named iTunes Library.itl. You can rename it or just delete it. Then go to the folder of previous iTunes Libraries in the same directory, and find the most recent iTunes Library.itl and copy the specific file to the iTunes folder by removing the date info from the file name.
Step 4: Turn off iTunes automatic upgrade
This step is necessary to avoid the auto updating of iTunes. Otherwise, all above attempt will be in vain.Choose iTunes > Preferences > Advanced. Disable the Check for new software updates automatically option and then click OK to save the setting.
If you don't wish to downgrade iTunes version, there are other workarounds here: "How to Run TunesKit Successfully with iTunes 12.6 on Mac".