Business Partner xxxxxxx is not available in ERP.

This error typically means the Sold-To partner id in your quote has a role=Prospect instead of role=Sold To.  Prospect accounts exist in SAP CRM but do not exist in SAP ERP, so you cannot mark a quote in CRM as Order Received when the Sold-To is a Prospect.

First option is to do a search in accounts and find a different Partner ID that is already setup as a Sold-To, then redo your quote, then mark it Order Received.

If there are no other Partner ID's already setup as a Sold-To, then you would need to have the Prospect promoted to a Sold-To.  You could go directly to the Master Data Team, or work with Dean, Daniel, Ari, or Michael to facilitate that.  

Click on this link to go to this Help Juice article:  How to promote a Prospect account to a Sold-To account