Unable to add AirTag to my backpack

Re: i have forgotten what item when I set it up so when I try to add it to my backpack and then it says it’s locked it is connected to my Apple ID and my phone number

A locked AirTag may be connected o another Apple Account ?

or

This discussion thread has a tip that may help:

How can I identify which AirTag belongs t… - Apple Community

Eg: See whether or not you named your AirTags ...

.(.. else you may be able to check which AirTag it is, at least.)

tap on the Find My app,

then tap on the Items tab.

Select the most likely one of your AirTags, and finally, tap on "Play Sound" to identify it.