The SIM currently installed from a carrier is not supported under the activation policy that is currently assigned by the activation server. Suggested that plz contact Apple and request that this phone be unlocked by your carrier. For more information please connect with Apple. (snap shot below)
[Edited by Moderator]
iPhone 8 Plus
Apple cannot unlock the phone. Only the carrier can unlock the phone.
The most important fact in the Apple Support document linked below:
How to unlock your iPhone for use with a different carrier - Apple Support
they don't unlock it unless given a go from the carrier it's locked to
How to unlock your iPhone for use with a different carrier - Apple Support
The SIM not supported.
