Quantity conversion of material &1 from UoM &2 to UoM &3 failed (RC=&4).

What causes this issue?

The UoM conversion (without ATSM) failed.
The return code here means:
1 Conversion not found
2 Input invalid
3 Material not found
4 Alternative UoM not found
5 Base UoM is missing in the material master data
6 Alternative UoM is missing
7 Output invalid
8 Overflow
9 Others

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.