Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
Hi
You will need access to the tenant admin account. You need to call the licensing support if the user that did quit didnt left you the necessary information to logon.
They can reset the admin account, but they need validation. (https://support.microsoft.com/home/contact)
The reseler can help too sometime, to change the email, but it's process that take time.
Thanks