Diagnosis
The BDoc Type cannot be deleted because inside an Adapter Objectdefinition there is a reference to it.

Procedure
Check if the BDoc Type shall really be deleted.
If it shall be deleted, the Adapter Object defintion must bereconsidered first.
Maybe the complete Adapter Object shall be deleted or at least thereference to the BDoc Type must be deleted. Afterwards the BDoc Typedeletion has to be done once again.