This Conga error message will occur if the end users have Restricted Field Level Security set for their profile. If the end users are trying to generate a document that requires data from certain fields or reports that they do not have access to they will see this error message. Restricted Field-level security settings let administrators restrict user access to view and edit specific fields.
In some cases there may be a change to security settings that can contribute to why some end users may have access verses others.
What to do
Contact your Salesforce Administrator to find out if there has been a change in the security settings.
For more information see Salesforce Field-Level security.