The most common cause of not being able to see the 'Mark as closed' button in your Optimiser, is that we are not able to find the message.
Please see the below scenarios where we would not be able to find a message:
The message is marked as spam and therefore we didn't ingest it.
The message is a draft and therefore we didn't ingest it.
It's a legitimate message, however, we haven't ingested it yet.
The message is too old, and therefore we didn't ingest it.
There is an ingestion issue for the particular agent and therefore we aren't ingesting anything for them.
After checking all the above possibilities, if you suspect that the issue is related to an ingestion issue as mentioned in point 5, please reach out to our support team for further assistance.