Someone else gained access to the iPhone device using another apple ID.

Let's hope the scammer wasn't clever or else you will be shopping for a new iPhone. You were unfortunately a victim to a common scam. By signing into the other account you basically gifted your iPhone to that account owner which they promptly blocked you from removing again.

● Apple might reset a device if you can prove to Apple's satisfaction you are the owner. However, Apple will not do so if Lost Mode is set on a device which may have been done by a scammer if a person signed into their device with the scammer’s account.

Apple can completely reset a device if a person can provide sufficient evidence of valid ownership. This is done online* at: Activation Lock support request - https://al-support.apple.com/#/additional-support Apple says, "Proof of ownership documentation is required," but does not provide many details in the article as to what is acceptable proof. Prior to this official service Apple typically required the original receipt** issued by Apple or an Apple Authorized Reseller.

* There is a special link for this service. To save time you should use it instead of https://getsupport.apple.com/

**Apple recommends you keep your receipt after you buy an Apple device. --> Keep your receipt after you buy an Apple device - Apple Support