Cannot load or replicate table &1 (key field issue); see long text

What causes this issue?

SAP LT Replication Server cannot perform an initial load for the table,
or replicate data for the table as there is an issue with the key fields
of the table. In order to perform an initial load<(>,<)> or to replicate
data, the following restrictions apply:
The table must have a primary key
The table cannot have any key fields that are defined as nullable.
The table cannot have more than 16 primary key fields.

System Response

SAP LT Replication Server cannot perform an initial load for the table,
or replicate data for the table.

How to fix this error?

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.