I cannot find the Seats tab in the license portal for my licenses to assign seats.
Confirm if you are using an Enterprise (unlimited seats) Subscription license. From the license overview page, the top section of the page show information about the license: Number, Site Message, Usage etc. If Usage is set to Unlimited Seats then your license is an Enterprise license.
Enterprise Subscription licenses with unlimited seats are configured in the license portal to automatically assign seats to end-users, eliminating the need for manual seat assignment by license administrators. Therefore, the Seats tab is not applicable to such licenses. See KB article 4381210 for additional details and how to configure such licenses.