... start parallel export, degree of parallelism: &

What causes this issue?

Parallel export starts

System Response

After the first export has been completed successfully, the following
transports are started in parallel. You can specify how many exports
are started in parallel.
A degree of parallelism of 1 means that the transport requests are
exported one after the other.

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.