Job cannot currently be started "Immediately"

What causes this issue?

A job is to be started with start condition "Immediately".

System Response

The system check determined that this is currently not possible. This
may be because there are no free background processing resources (that
is BTC work processes).

How to fix this error?

You can configure additional background work processes using OPMODE or
the profile.

Procedure for System Administrators

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

Error message extract from SAP system. Copyright SAP SE.