Holder has exceeded maximum number of allowed accounts for product

What causes this issue?

This account holder has already got &v1& active accounts from the same
product type. The definition of the feature in the product (maximum
allowed: &v2&) specifies that no additional accounts from this product
type may be assigned to or opened for this account holder.

How to fix this error?

Before this account can be opened for this account holder, you need to
delete at least one of the other accounts in the same product type.

Error message extract from SAP system. Copyright SAP SE.