Every time I put this card into the P4P I get an SD card error. I can't format it in the DJI app.

The supplied 16Gb card works ok, and the Sandisk 64Gb works okay in my PC, however it refuses to work in the Phantom, Any ideas?

The card is formatted as ExFAT

I have the 64GB Sandisk Extreme Pro, works like a charm, but you mentioned a Sandisk Extreme.. hmm try a low level format on your pc, making sure everything is set up on Default and format. After that, you also would want to format in the P4P itself.