You need to go into the SWPM install directory and run ./sapinst. The reason for the ./ in the front is the current directory might not be in your PATH and thus the sapinst executable will not be found. Another way to resolve this would be to put ./ at the end of your environment variable PATH which would solve the ./ issue on any other instance where you want to run an executable within the PWD without the ./
Cheers,
Thusjanthan Kubendranathan