Universal ID is enabled for your Marketo instances, but when you log in, you are not able to switch between instances.
First, make sure you are logging in with the login used for the Universal ID. If your old, pre-UID logins were not removed, they will still allow you to sign in on the instance they were associated with.
If the login is your primary login that should have been associated with the Universal ID, please have an Admin user remove your login and reinvite you to all the instances included in the Universal ID.
Who This Solution Applies To Customers using Universal ID