Hello Dermot Keane,
As per the screenshot, you have downloaded and installed the Windows AxCrypt app version. that is the reason you cannot install the windows AxCrypt app version in the Mac system.
Please download the AxCrypt app mac version in our official website(https://www.axcrypt.net/download/ ) filename(AxCrypt-mac-pkg)file.
After downloading the AxCrypt app Mac version, then install the AxCrypt app and follow the steps previous forums link.
Still, you are facing any issue, please provide more information about the issue and write a mail to firstname.lastname@example.org .