I am having trouble adding a second id to my apple tv

Lets try this,

Make sure your apple tv has the Latest software running!

If its already running the latest software then it could be two things,

1. Verify you Apple ID first by signing in to this link :- https://appleid.apple.com/

2. Restore the Apple TV to factory default settings.

This article will help:- https://support.apple.com/en-ca/HT202118

"Hope this helps"

What exactly are you trying to add it for. You can only use one ID for iCloud and home sharing, but you should be able to add multiple ID's for the iTunes store.