Why can't I add a contact to a product?

If you add a contact to a product they're not eligible for, you will see a reason in the "Add to product preview". Here's a list of all the reasons and how to fix them

PRODUCT_NOT_ALLOWED

• An unfulfilled product relationship is blocking the purchase
  • Add the contact to the required product first
• A merchandise product is out of stock
  • Set the merchandise stock levels in Settings > Products > Details
• The product is hidden
  • Check the Product settings, under details to make sure the product isn't hidden:

PRODUCT_NOT_AGE_APPROPRIATE

• Contact does not fit the age restrictions

CHILD_PRODUCTS_NOT_ALLOWED

• There is a required product relationship that the contact is not eligible for

ALREADY_SIGNED_UP

• The contact is already on the schedule for the product
  • If you need to bill again for this product, you can choose the option "None, just raise the invoice" to raise an ad hoc payment separate from the schedule