Alright so there is the issue.
When you log in with sc.omniture.com, you are actually logging into the Analytics only authentication system. Since the Experience Cloud Admin Console is only accessable by users that are logged in via their Adobe/Enterprise/Federated IDs, the system is asking you to log into the Experience Cloud.
What you likely need to do is log in via marketing.adobe.com and navigate back to Analytics via the solution switcher in the upper right. Once you're back into Analytics using your Experience Cloud login, you should see the migration tool.
As a side note, your Analytics login might use the same username and password as your Experience Cloud login, but they are not the same system. Similar to how you could use the same username and password on two different websites.