iphone 7 plus without imei on sim tray

Here is where it should be:

Find the serial number or IMEI on your iPhone, iPad or iPod touch - Apple Support

If your iP7+ was a second hand or refurbished device, perhaps the SIM holder was replaced.

Regards

Giulio

Used Iphone? The seller should have informed you.

Where did you buy it? If not at an Apple Store nor at an Authorized Service Provider, that could happen.

Refurbished iPhone? If it was refurbished by Apple, its Model Number should start with letter "F"

• M – Brand new device, meaning the device was purchased new
• F – Refurbished device, meaning the device has been through refurbishing process (by Apple).
• N – Replacement device, meaning the originally bought device was replaced by this model likely due to a service request

Check: Settings>General>About>Model

Then re-open: Settings>General>About>Serial Number... and report that number here, to check the Warranty: Check Your Service and Support Coverage - Apple Support

Regards

Giulio