All PBT resources are currently busy

What causes this issue?

The system tried to initialize the environment for processing parallel
RFCs using function module SPBT_INITIALIZE. Here it was established
that there are currently no resources in the system that are necessary
for processing parallel RFCs. This is a temporary problem.

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?

There are several ways to solve this problem:
Wait until the resources are released by other programs
Increase the number of dialog work processes on the application servers
that process the parallel RFCs
Install additional servers in your system or include new servers in the
server groups for parallel RFCs (see Transaction RZ12).

Error message extract from SAP system. Copyright SAP SE.