This issue can be caused by custom CSS in Altru's page designer. Adding a certain tag in Altru's text editor like <Form> can cause this issue. This is because Altru's web forms are built on <Form> tags. Try removing your CSS from these places to see if it is causing the issue:

Page Designer Text Editor: 
  1. Click Web > Page Designer
  2. Click on the Colors & Styles tab > Click Advanced Colors and Styles
  3. Click edit style sheet
  4. Try copying your CSS code from here and pasting it into another text editor like notepad so it is not lost. Then delete all of the CSS code and click apply. Then click save and submit the changes. After the changes are submitted go to your membership web form and try to add a membership to the cart. If you are able to add a membership to the cart then there may be an issue with your CSS code. 
HTML on the Content Page:
  1. Click Web > Page Designer
  2. Click Next > Click on the Configure Content tab
  3. Click on your web form's banner at the top of the page
  4. Click on the View tab > HTML code
  5. Search the code for any <Form> tags and remove them if found
  6. Click Update > click save
We are unable to provide any assistance with the actual CSS in your page designer as this is outside the scope of Blackbaud Support. You may need to reach out to your IT team or a web designer to determine the portion of code that may be causing the issue.