In AR Customer Maintenance, the "Relate to CRM" checkbox cannot be unchecked if there are orders that are referencing SageCRM Opportunities in the order entry file. That is where I would look first.
The second part of not being able to select an existing opportunity can be related to different scenarios, but I hope this is helpful:
In Sales Order entry, does the "** SageCRM Customer **" message display on the screen (in red)?
If so, and the SageCRM Opportunity is blank, please confirm the Contact field matches the Person selected for the opportunity in SageCRM (including blank). It must be an exact match.
If not, the quote may have been entered for the customer while it was not linked to a SageCRM company. If you link an existing customer to a company in SageCRM, existing orders/quotes are not updated with the link information and unfortunately cannot see the opportunities available.
