I am getting an error message saying this is not available in my region/country

This offer is available to all members of Anytime Fitness Australia who reside in Australia and have a compatible device. If you created your Apple account in another country OR do not have an address listed on your Apple account, the activation of your Fitness+ trial will not work. 

To update your address to an Australian address, please follow these steps: https://support.apple.com/en-au/118283