ms-access concat a string with control source
As the users in the comments above pointed me in the right direction, the issue was a circular reference error between the name of the created field (TextBox) and the Control Source. These can be viewed in the Property Sheet of a form or report under All tab. Obviously you don't want to rename the Control Source between this references the table or query the data's being pulled from, so just change the name of the text box. For whatever reason MS Access defaults with the names being the same so this is a common issue. In the image, originally the fields Name and Control Source both contained Insert.