No PBT resources available in system

What causes this issue?

The system tried to initialize the environment for processing parallel
RFCs using the function module SPBT_INITIALIZE. Here the system
established that there are no resources in the system. This is a
configuration problem. Only those application servers with at least
three dialog work processes can be used to process parallel RFCs.

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?

If no specific server group name is specified for processing parallel
RFCs:
All application servers in your system have less than three dialog work
processes. If you want to process parallel RFCs, at least one
application server must have at least three dialog work processes.
If a server group name was specified using Transaction RZ12 for
processing parallel RFCs:
All application servers in the server group have less than three dialog
work processes. If you want to process parallel RFCs, then at least one
application server in the server group must have at least three dialog
work processes.

Error message extract from SAP system. Copyright SAP SE.