Conversion routine & does not exist

What causes this issue?

The specified conversion routine does not exist.
Conversion routines are implemented with conversion exits, which are
stored as function modules. Each conversion routine has a five-place
name. This name, for example xxxxx, also appears in the name of the two
function modules which have to exist for this routine:
CONVERSION_EXIT_xxxxx_INPUT
CONVERSION_EXIT_xxxxx_OUTPUT
The specified conversion routine does not exist if:
- one or both conversion exits do not exist
- or the name of the conversion exits does not satisfy the above name
convention.

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?

Implement both conversion exits for the specified conversion routine.
If these function modules already exist, check whether the names
satisfy the name convention for conversion exits.

Error message extract from SAP system. Copyright SAP SE.