The reasons for this situation are:

1) The region of the purchased gift card does not match the region of your own account

2) The gift card has already been used.

You have to contact customer service, provide the following information:

Some specific screenshots are crucial for the seller to properly investigate the case. Remember to attach the following ones:

1) Entered key before activation

2) Error message

Make sure that all your screenshots are unedited and your taskbar with time/date is visible.

Then seller will check and fix your problem.