Receiving email validation error 'HTML_Body - Substitution_String_Missing_DE_Field'

I have created a journey based on a data structure that we have set up on audience builder. It pulls through two fields into the email, one called forename and the second called login code. When I try putting the email into the journey builder, I receive the following error:

HTML_Body – Substitution_String_Missing_DE_Field

It pulls through the forename fine, but the login code is the issue field it seems. I have checked the data structure and this code is in the same table as forename so don’t see any reason for this to fail. I double checked all the emails and they are fine too.

Any ideas on what could be causing this?

Thanks