The file "iTunes Library.itl" cannot be read error because it was created with a newer version of iTunes (12.7.4 update)
After hours of research to no avail, solution:
1. in preferences tell it where the library is, in my case, /Volumes/Hard Drive/iTunes/iTunes Media (Hard Drive is name of my external storage device)
2. hold option key while opening iTunes
3. select Choose Library...
4. select /Volumes/Hard Drive/iTunes/iTunes Library.itl
5. the solution is to select iTunes Library.itl!!!!
iMac, iOS 11.3, null