That depends on where you're running adb from.
If your adb.exe is located in C:\adb\adb.exe, you probably navigated to C:\adb first in the command prompt. which means that when you only type mmcblk0p21_persist_ext4.img, that it's referring to that img file in C:\adb. So make sure you place the files you need to push to your phone in the same folder as adb.exe.
If you didn't navigate to a certain folder first, that means that your adb is installed globally on the system (and it's probably located in your windows folder or your environment variables have been changed). In that case the easiest way to fix this issue is by creating a folder on your C:\ drive called adb (any folder will do as long as there aren't any spaces in the name), place all the files you have to push in there and use the command adb push C:\adb\mmc_blk0p21_persist_ext4.img /sdcard/. Simply add repeat that (adding C:\adb\ before the filename) for the other files as well and that should resolve it.
Let me know if you figured it out