Problem with Iphone 6s Plus

Are you getting a message similar to this when inserting an American Sim card?:

If so, your iPhone may be locked to a Croatian mobile carrier. If it is, only that carrier can unlock it for use on other carriers.

How to unlock your iPhone for use with a different carrier - Apple Support

If you can't activate your iPhone - Apple Support - https://support.apple.com/HT201407 - "If your iPhone can't be activated or an alert says the activation server is unavailable or the SIM card is unsupported, learn what to do."