Hello there, countyaccess. I hope you're doing fine today.
Once the previous master admin is available, they can transfer the role to you. Since they're no longer working with you, you can send a request to be the master admin or primary contact in the QuickBooks Online account. You'll be asked to gather a few information and documents for verification purposes.
Once done, you'll need to enter the details and upload the documents you have using this link. As we can see, there are types of information you'll need to complete and make sure they're all accurate. Then, click the Next button.

You may want to check this article for more tips and details about the user types and permission in QuickBooks Online: Learn about the different user types and levels of access in QuickBooks Online.
Please don't hesitate to comment below if you have other questions. Take care always.