This will occur if the user is editing a page that already has a Membership Form in place. Only one can be present on a page at at time.

If you don't see the part initially, while editing the page, check the following areas:

Page
  1. Navigate to Site Explorer>Pages & Templates
  2. Click the Search tab
  3. Search for the name of the page you're trying to insert the membership form on
  4. Once you find the correct page, click on it in the upper pane to highlight it
  5. In the lower pane, confirm if a Membership Form part type exists
    • If a Membership form part is here, you will not be able to insert a duplicate
    • If you see a Membership form part here, but you don't see it while Editing the page, click the Scissors icon to cut the part, then insert the part back onto the page.
Template
  1. Navigate to the page you want to insert the membership form part onto
  2. Click Edit this page
  3. Click the Template Designer Button
  4. Do you see a Membership form listed here? 
    • If a Membership form part is here, you will not be able to insert a duplicate
    • If you see a Membership form part here, but you don't see it while Editing the page, click the Scissors icon to cut the part, then insert the part back onto the page.