Full Error Message:
rsdoc.OpportunityGenerateDocument: execution of AfterUpdate caused by: System.DmlException: Insert failed. First exception on row 0; first error: REQUIRED_FIELD_MISSING, Required fields are missing: [rsdoc__Document_Action__c]: [rsdoc__Document_Action__c] Trigger.rsdoc.OpportunityGenerateDocument: line 48, column 1

App Expert Reply:

When you are creating an opportunity, if you are specifying the RS Document to Merge field value, you have to set it as a document action name (for example GDT-000000), and you must have a document action with that name in your org.
Also when you are creating or refreshing developer or developer pro sandbox, document action records are not copied, so you need to create them. And the IDs will also be different.

Other feedback from specific customer case:

One specific reason that you may see this error is if you refreshed a developer sandbox, the Document Actions are not automatically copied from production to  sandbox environment. However, the automation workflows responsible for assigning proper Document Actions to RS Document to Merge parameter are copied to sandbox environment. This caused a conflict: RS Document to merge could not find the Document Action it was assigned to.
The above automation with workflows is described in the first 3 bullet points of this article about automation.
In addition to the above, when manually creating Document Actions on sandbox environment, the Document Action ID parameter cannot be edited manually (it is always assigned automatically to the next consecutive number) and in this customer case, it did not match the Document Action ID the workflows were referring to.
So for this case, after refreshing a developer sandbox, they need to manually adjust the field update workflows responsible for assigning RS Document to Merge, to point to the proper Document Action ID’s (valid for this particular sandbox).