Do you see an error message while adding 'Credits'? Also, here's more info about it: https://goo.gl/o7xhss . Let us know and we'll take a look.
Hmm, have you tried these steps: Open the video you’d like to edit > On the basic info tab, scroll down to the "Video Credits" section > Add a role and identify the collaborator by typing in their YT username or by adding the channel URL > Once done, click Save changes?