Picture of the issue.

Currently receiving the error - 'Unable to check for errors. Go to the Designer tab and try again.' however, the designer tab lists no errors.
Preview section displays 'There was a problem with one or more expressions in the HTML. Displaying preview without data. For error details, click the "Check content" button.' so it is a bit of a blocking circle as I can't do one without the other.
I know in the picture too that it says there is no subject or sender, however, this is not the case as these fields are completed.
What I have tried so far is removing the A/B testing, duplicating the mailer using save as, and the usual sign out and back in.
Any suggestions? Thanks