Conga Contracts uses Salesforce Files to store documents in Salesforce, which replaces the older Attachments object. Salesforce Files provide more robust features, such as versioning, which is required by Conga Contracts.
Lightning experience users will always create Files instead of Attachments. However, users of Salesforce Classic may still be using Attachments when adding documents to records.
To remedy this, we recommend enabling the Files uploaded to the Attachments related list on records are uploaded as Salesforce Files, not as attachments setting under Salesforce Files > General Settings. This will ensure Salesforce Classic users create Files to prevent attaching documents that cannot be used with Conga Contracts.
This setting is on by default in newly created Salesforce orgs.