iPod touch 6th from iOS 12.5 back to iOS 8.4

Ouch! Backwinding typically is only possible between the next younger version and limited to about a year when the embedded certificate expires of older release. Apple does this to compel people to move forward.

In your case the best you might be able to do is if you had backed up under the older 8.4 before upgrading. Then you should be able to restore back. Even still that might not work as the jump here is quite sizable as such the newer iOS firmware may prevent it.