Gallery App permissions error when trying to play videos stored in SD card?

[AC Question]

A friend of mine who doesn't speak the English well is having trouble getting videos he's taken to run on the "Gallery" app on his device. They're currently stored on his SD card instead of local storage. When he tries to play the videos, he gets a "You turned off permissions for the storage. To edit, go to Apps Settings." error message. I've checked the apps settings on his behalf, and it looks like all the permissions are currently allowed. Can anyone shed some light in this issue?

Device: Samsung Galaxy S5
Android Version: 6.0.1

Thanks, any help is greatly appreciated!