Kindly confirm the exact AEM version you are working with, configuring AEM Assets with Brand Portal via Adobe I/O is supported on AEM 220.127.116.11 and above and AEM 6.5.4 and above. So, make sure you've got the latest service pack installed on the instance. Also, login as administrator on the instance to have sufficient rights for making the config.
If anyone is looking this up and is looking for a solution. I found one.
I am on AEM 6.5.6 and if a brand portal config doesn't appear it might be because a config is already created under Tools -> Cloud Services -> Legacy Cloud Services. You can also check if a config is made by going in /crx/de and seeing if there is a config under /etc/cloudservices/mediaportal. If there is, you can remove the config by either deleting it or replacing it using a package. This fixed it for me.