Try formatting the SD Card while inserted in the phone.

1. Go to device “Settings”, then select “Storage”.
2. Select your "SD Card", then tap the “three-dot menu“ (top-right), now select “Settings” from in there.
3. Now select “Format as internal”, and then “Erase & Format”.
4. Your SD Card will now be formatted as internal storage.
5. Reboot your phone.

Reference: http://www.tomsguide.com/forum/id-3325411/download-due-card-missing.html

Try ERASING (reformatting) the SD Card while inserted in the phone.

0. Back up your data
1. Go to device “Settings”, then select “Storage”.
2. Select your "SD Card", then tap the “three-dot menu“ (top-right), now select “Settings” from in there.
3. Now select “Format as internal”, and then “Erase & Format”.
4. Your SD Card will now be formatted as internal storage.
5. Reboot your phone.

Reference: http://www.tomsguide.com/forum/id-3325411/download-due-card-missing.html