> I am unable to add new contacts in my contact list.

We can't tell from this description if you are trying to add contacts to a mailing list, or to an address book.

When an address book fails to retain new contacts or edits, these steps usually fix it: right-click Personal Address Book in Address Book, Export..., choose LDIF format, save the file. Help/Troubleshooting Info, Profile Folder, Open Folder, close TB, delete or rename abook.sqlite. Restart TB, in Address Book, Import the LDIF file (click Import icon at bottom of left pane), then select the contacts in the imported address book and drag and drop them onto the Personal Address Book icon in the left pane. See if PAB retains new contacts and edits.

I can't find either ADDRESS BOOK or CONTACTS. Notes I have seen say blithely to go to them, but how do I do that? I find no trace of either in menus or settings. Using MacBook Pro, OS 13.7.8. It's simple I'm sure. That's the level I need!