my computer displays message mac os cannot be installed because computer is running encryption

Hi,

Did you turn on FileVault or erase and format your startup disk as encrypted? If so, you'll have to wait for it to finish, usually hours. I'd give it overnight and try again.

Filevault state, open Terminal & paste these in & press enter…

    fdesetup status

    diskutil cs list | grep 'Conversion Progress'