Value "&" is not allowed for characteristic "&"

What causes this issue?

You have selected an invalid characteristic value.
In order to ensure that the plan structure is consistent, the system
requires that all the nodes that lie directly below the same node (at
the same level) represent different characteristic values of the same
characteristic.
In addition, a characteristic can only occur at one level of one branch
of the hierarchy.

System Response

This characteristic value is not allowed.

How to fix this error?

Check your structure again and, if necessary, select a different
characteristic value.

Error message extract from SAP system. Copyright SAP SE.