Hi Lo,

I'm sorry to hear that you're having trouble adding a new sheet to your Excel workbook. Here are a few things you can try to resolve the issue:

1. Check if the workbook is protected: If the workbook is protected, you won't be able to add new sheets. To check if the workbook is protected, go to the Review tab and click on "Protect Workbook". If the "Protect Workbook" option is grayed out, the workbook is not protected. If it's not grayed out, click on it and enter the password if prompted.
   
2. Check if the sheet is protected: If the sheet you're trying to add a new sheet to is protected, you won't be able to add new sheets. To check if the sheet is protected, right-click on the sheet tab and select "Unprotect Sheet". If you're prompted for a password, enter it and click "OK". If the sheet is not protected, the "Unprotect Sheet" option will be grayed out.
   
3. Check if the workbook is shared: If the workbook is shared, you won't be able to add new sheets. To check if the workbook is shared, go to the Review tab and click on "Share Workbook". If the "Share Workbook" option is grayed out, the workbook is not shared. If it's not grayed out, click on it and uncheck the "Allow changes by more than one user at the same time" option.
   
4. Check if the sheet tab is hidden: If the sheet tab is hidden, you won't be able to see it or add new sheets to it. To check if the sheet tab is hidden, right-click on any sheet tab and select "Unhide". If the sheet you're looking for is listed, select it and click "OK". If the sheet is not listed, it may have been deleted.

I hope one of these solutions works for you. Let me know if you have any other questions or if there's anything else I can help you with.