Have you tried renaming a sheet and Excel just doesn’t take the ‘name’?

Well, Excel has sheet naming rules, and they include:

• A sheet must have a name, even if you leave the default name.
• Sheet names are limited to 31 characters.
• There are a few special characters—[ ] / \ ? * :—that you can’t include in a sheet name.
• Don’t start or end a sheet name with the apostrophe character (‘)
• You can’t use an Excel reserved word (such as, History)