First I need to verify you meet the requirements to use the PURL:
- You must purchase Landing Pages.
- You must contact you Salesforce Marketing Cloud to activate the PURL feature.
- You must have a domain or subdomain configured in your account specifically for landing pages. However, the domain must not be identical to the domain used for SAP, but it may be a subdomain of the SAP. The client can use the same domain as their website as long as "pages." is at the beginning of the domain. For example, if the domain used for SAP is myaccount.com then the client could use pages.myaccount.com for the landing pages.
If you meet those requirements, then I believe the issue is that you need to add the email address or subscriber key after the last forward slash, not as a parameter.
Add the Email or Subscriber Key substitution string to the end of the
URL by including %%emailaddr%% or %%_subscriberkey%% into the URL
after the last forward slash.
alias="Join Us">Join Us</a>