It would help if you said at which stage (in the list supplied) the pairing failed, stage 4 ? , If the 64sx has displayed a passkey then the phone and the GPS have 'seen' each other, I'm guessing that the problem is that the phone does not accept the number that has been supplied by the GPS ?, there is a time limit when entering the 6 digits
The phones Bluetooth pairing page will have a list of 'paired' devices and a list of 'Available' devices (not yet paired), you need to select GPSMAP 64sx from the available list while the GPS is displaying 'Ready to Pair', this screen will change to Passkey when the two units see each other, after the 6 digits are entered into the phone it will either move GPSMAP 64sx to the 'paired' list or it will remain in the available list, selecting GPSMAP 64sx again from the available list will prompt the GPS to display a new Passkey