Invalid data type & - Choose a valid data type

What causes this issue?

A container element can only refer to elementary data types with
variable lengths (such as <AB>CHAR</> or <AB>NUMC</>). Data types with
a fixed length should be designated ABAP Dictionary types.

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?

Create an appropriate data type for this data element in the ABAP
Dictionary and enter it.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.