Did you give AirDroid the permission to take screenshots when you installed it?
If you didn't give it the permission, this problem might occur. Would you please reinstall AirDroid and give it the permisssions again for a try?
Screenshot feature may not work well on Android 4.4.x. We've figured out the cause and have made some fixes. Hope the fixes work for you.
If you do need this feature, please contact support@airdroid.com to get the workaround on this.