Hey there! It sounds like the workbook may be protected, which is why you're unable to insert new worksheets. Here's what you can do to check and remove the protection:

1. Go to the "Review" tab on the ribbon at the top of the Excel window.
2. Look for the "Protect Workbook" button. If it's highlighted, the workbook is protected.
3. Click on the "Protect Workbook" button and select "Unprotect Workbook" from the dropdown menu.
4. If the workbook is password-protected, you'll need to enter the password to unprotect it.
5. Once the workbook is unprotected, try inserting a new worksheet again by right-clicking on an existing worksheet tab and selecting "Insert" from the dropdown menu.
6. If the "Insert" option is still not available, try going to the "File" tab, selecting "Options," and then selecting "Customize Ribbon." Make sure the "Developer" tab is checked and click "OK." Then, go to the "Developer" tab and try inserting a new worksheet from there.