Database connection &1 does not exist in table DBCON

What causes this issue?

There is no entry in the table DBCON for the selected database
connection.

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?

An entry must be made in DBCON so that a connection can be made to
database.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.