Using SD card from Android 7 in Android 6
Try to connect your old phone to your computer and copy the data from the SD card to  the computer. Then choose forget on your old phone, try to get the SD card out and put it in the new phone. It will prompt to format the SD card. If it doesn't, go to Settings > Storage and tap on the SD card. It will ask to format it. If that still doesn't work, put your SD card into the computer and format it from there, preferably FAT32.