How to fix "There was an error adding the new plan"

Cause #1: Shopify displays that error message if the frequency of the delivery and billing doesn't match up.

Solution: Shopify likes it to be week/week, month/ month, year /year.

For example, if you are trying to do monthly billing with weekly deliveries I would try: delivery every 1 week/ billing every 4 weeks.

Cause #2: Shopify displays that error message if the plans are too similar (for example if the plans are both delivery and billing every 1 month but have a different expiration date)

Solution: if that's the case, you'll want to create a new subscription group for your new plan. If the new subscription plan is in a new subscription group, you will avoid this error.