Are you running into trouble finding your channel on Google when trying to add it to Thumbnail Test? If so, the problem likely lies with one of two things:
Permissions
Youtube only allows channel owners to add it to third-party tools.
Even if you have permissions as a Manager or editor, only the owner can add it.
For how to add a channel you don't have owner access to you can read this article: Agency Setup on Thumbnail Test
Channel Not Set up as a Brand Account
Youtube channels need to be set up as brand accounts and the correct permissions need to be set for the brand account in order for Thumbnail Test to use it
You can read about Brand Accounts here: https://support.google.com/accounts/answer/7001996?hl=en&co=GENIE.Platform%3DDesktop&oco=0#zippy=%2Chow-brand-accounts-differ-from-regular-google-accounts
Other users were able to find Brand Account settings here:
YT Studio > Settings > Permissions > Manage Permissions
