Index name &1-&2 is only significant in the &3 position

What causes this issue?

This table already has an index whose first two places are the same as
those of the selected index name.
Tables with 15- and 16-place names must have index names whose first
two places are significant in order to be able to generate a unique
index name on the database.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Choose another index name. If an SAP index collides with a customer
index because the customer index was not created in the customer name
range, you should delete the customer index and create it again with
another name.

Error message extract from SAP system. Copyright SAP SE.