Android - Unable to take screenshots, "Couldn't save screenshot, storage may be in use" error

It might probably due to permissions error in KitKat (and Lollipop) running Android phones.

♦ Layman Solution :
The most common way is to reboot your android phone and try taking the screenshot.
If it doesn't cut it, try going into safe mode and come back to normal mode back.

♦ Professional Solution :
Wipe dalvik cache in recovery mode.

♦ For Xiaomi users : Create a folder called Screenshots in /sdcard/DCIM/
Note: MIUI saves screenshots in /sdcard/DCIM/screenshots/