When you install the Docentric replica model using the Docentric Free Edition, you gain access to various features that enhance the user experience in D365FO. These include out-of-the-box report-specific placeholders and improved print archive and emailing options. To understand the benefits of the replica model in conjunction with the Docentric Free Edition, I recommend referring to the manual How to Use Docentric Replicas With Free Edition.
However, it’s important to keep in mind that the Docentric designs included with the replica model cannot be used with the Docentric Free Edition. This is why you are encountering the error. To resolve it, set Use only print destinations to Yes in the Docentric report setup for the specific report resulting in the error.
Adding additional email attachments through the user interface is included only in the Full Edition version. The Email an invoice with additional attachments from D365FO article you linked in your comment describes the process through the user interface. With that in mind, as a Docentric Free Edition, you cannot take advantage of the process described in this blog post.