To be able to connect you need Admin permissions in Criteo. If you're unable to select an advertiser it's because you don't have Admin permissions for that specific advertiser.
You can check and revoke access you have given at the Criteo Consent Portal.
Setting up new Credentials
Press either reconnect or connect a new Criteo source, then press New Credentials.
The next step will take you to Criteo's consent page. First login to Criteo:
Once logged in you will be presented with a list of advertisers to grant access to:
If the list of of advertisers is greyed out this means that the user does not have the necessary privileges. Please contact Criteo to get administrator privileges.
Also note that advertisers that other credentials has already been granted access to will not be included in the list.
Please select the advertisers that this set of credentials should have access to.
Then press Approve.
You should now have new set credentials for Criteo.