This article will explain why you are receiving "Can't add MD5 records to a plain text list" message and how to fix it.
Why am I receiving a "Can't add MD5 records to a plain text list" message?
This message is triggered when you are trying to upload MD5 email records to a list that has the storage settings enabled for plain text and MD5. This means we can distribute the data on this list in either "Plain Text" or "MD5". Since the emails you are trying to upload to the list are already in MD5, we prevent you from uploading it because we don't have the plain text version available for future downloads. To learn more about MD5 vs Plain Text lists, review this article: https://support.optizmo.com/hc/en-us/articles/360024335394-MD5-vs-Plain-Text
How do I fix this?
If you don't have access to the Suppression List in plain text then you will need to create a MD5 only Opt-Out List, and then create a Group List, so you can combine your existing list with the MD5 only list. Here is a article that speaks to how you can setup a MD5 only list and a Group List: https://support.optizmo.com/hc/en-us/articles/360005021134-Creating-an-Opt-Out-List
Once the Group List is created, an OPTIZMO administrator can switch the Opt-Out List for your existing campaign with the Group List. Please contact: email@example.com for assistance.
If you have any further questions, please contact: firstname.lastname@example.org