The NFC antenna and coil assembly might get stick to the back cover due to adhesive spreads while trying to pry the back cover to open the phone. Take a look inside to ensure the assebly has not got sticked. If so, try using a credit card like plastic to get it peeled of from back cover without breaking the assembly always paying attention to the fingerprint sensor as well.

I had to pry off the NFC antenna and charging coil assembly from the back cover. The adhesive probably spread a little bit while prying around the phone sticking some part of the NCF antenna and coild assembly sticked to the back cover. I had to use a credit card like pice of plastic to get it unsticked from the back cover without bearking it.

When prying the battery off at the first battery position shown in the picture (the gap between battery socket the battery itself), a small rounded metal spudger (like the one that comes with the iFixit Pro Tech Toolkit) is recommended if you break your plastic prying tool (like happens to me) during this step. Just make sure that when prying up, you are not touching either the battery socket nor any other logic board component.

I also recommend to use a tool like the iOpener and put it on top of the battery for few minutes to reheat the battery and soften the battery adhesive during this step.