No idea what IDoc this is about, but I'll go with an assumption that this is an inbound ORDERS IDoc.
You cannot "re-use" the IDoc and you do not just "generate new order number". You can only create a new IDoc and it will, in turn, create new order (if all goes well). I have a feeling that you might be doing some steps blindly, without understanding what they actually mean.
If you have a succesfully processed IDoc then for testing purposes you can use WE19 to create a new IDoc as a copy of the existing one. Depending on your configuration, it'll either be processed immediately (which I recommend for testing) or you'll need to use BD87 to process the newly created IDoc.
The other message is likely just a consequence of this whole story. Obviously either the IDoc was not processed (you should have status 53 for the successfully processed inbound IDoc) or you got a wrong order # or there was an update termination. Or the order actually could've been simply deleted.