Can't enable lost mode or find AirTag I know is either in house or yard. Says error no matter what I do..

Hey Nag2181,

We'd be happy to help out. If you're unable to communicate with or locate your lost AirTag, this likely indicates that its battery is dead or it's not near any other iPhone's in order to be located. Since it sounds like it should be nearby, it's more likely that the battery is dead on it. If you do find it, we'd suggest replacing the battery.

The following steps walk you through enabling Lost Mode on it, but it won't come on until it has power again:

Turn on Lost Mode for an item

To mark an item as lost, you need to turn on Lost Mode.

1. Tap Items at the bottom of the screen, then tap the name of the lost item.

2. Below Lost Mode, tap Enable.

3. Follow the onscreen instructions to enter a phone number where you can be reached. To enter an email address instead, tap “Use an email address.”

Important: Make sure you allow notifications for the Find My app. See Change notification settings on iPhone.

4. Tap Activate.

This is outlined further here: Mark an AirTag or other item as lost in Find My on iPhone - Apple Support

Thanks for using Apple Support Communities.

Take care!