That option has been disabled on the A21. I've had the same problem for months now and am LIVID that someone, likely Verizon so they can charge you for a phone with more internal storage. I've got a 250GB SD card that's useless other than manually copying photos and files over. But I HAD, and would like back, something like 300 Apps that take up more internal memory than I have, so I've had to remove them and keep a list of what needs to be reinstalled IF I ever get this fixed. I've talked to Samsung Tech Support several times until they gave up and suggested I take the phone to one of these new retail "Fix-It" businesses and have the operating system reinstalled, though no guarantee that will fix it, plus I can install the OS myself. The only sure-fire way I've heard that will work is to root your phone. Then you can use Apps like one I've used for 10 years or more called Apps2SD. I tried what one guy said would fix it for sure, which was using ADB commands with your phone connected to a PC. That didn't work either. NOT happy with a brand new phone that I don't have full use of.