Registration shortcodes don't seem to work

Hi,
I added a the registration shortcode for the membership level I am adding to the page at

https://piratesofthepublicdomain.com/148-2/

but it doesn’t show

(The “register:” bit is mine – I just typed it to be able to see if anything was showing up at all)

After reading other support queries, I also tried adding it to the page I am trying to protect at:

https://piratesofthepublicdomain.com/pirates-of-the-public-domain/

As you can see – that’s blank too.

The registration shortcode I am using is: [ms-membership-register-user membership_id=”XXX”]

Any idea where I am going wrong?

Thanks

https://www.ads-software.com/plugins/membership/