There are three main reasons why this could be happening:
You are using an outdated version of Excel.
Refer to the following article to check which version you are running and, if needed, to update your version of Excel: https://myeducator.freshdesk.com/support/solutions/articles/14000110267-microsoft-excel-add-in-checking-excel-version-downloading-updating-office-365
Your computer’s operating system needs to be updated or it does not meet the minimum system requirements.
Refer to the following article to see the minimum system requirements needed: https://myeducator.freshdesk.com/support/solutions/articles/14000050025-minimum-system-requirements
Your computer’s preferred browser needs to be updated.
For Macs: Safari
For Windows: Edge (Chromium), Edge, Internet Explorer 11
If after checking and updating these three things the blank pop-up window still appears, please reach out to the MyEducator support team at support@myeducator.com.