Reassigning not possible since maximum of 20 exceeded

What causes this issue?

A maximum of 20 items can be under one another as a result of the data
structure in which the financial statement version is stored.
For this reason, you cannot reassign as desired.

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?

Error message extract from SAP system. Copyright SAP SE.