If you get stdio.h not found errors, ensure your "Folders/Extensions" path is pointing exactly to the folder containing the bin , include , and lib directories.
Click Add another ARM Compiler Version to the List and browse to the folder where you extracted the files. keil arm compiler version 5 download
If you see "FlexNet" or license errors, ensure your MDK license is active. The standalone compiler checks for a valid Keil seat. If you get stdio