Document contains items for 2 externally managed warehouse numbers &1 &2

What causes this issue?

You cannot carry out <GL:stock_transfer>stock transfers</> which
involve 2 external <GL:warehouse_number>warehouse numbers</> using a
one-step stock tranfer method.

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?

Carry out the stock transfer in 2 steps:
In the first step, remove the stock from the first external warehouse
number using the appropriate single-step <GL:movement_type>movement
type</>.
After the removal step has been confirmed, carry out the putaway for
the warehouse that is to receive the stock using the corresponding
single step movement type.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.