The main reason you cannot add a hub to a membership is if it is not published - aka it's in draft mode.
To add a hub to a membership, ensure first it is published by going to Hubs - and then clicking on the 3 dots to open the hub settings:
Now on the hub edit screen - look at the top right of the screen and you should see a "Publish" link:
Click this button to publish your hub. Then go back to your membership and you should find this hub can be added successfully.