Checkboxes don't typically have a name that begins txt. Are you sure it is a checkbox and not a textbox?

The type of the userform textbox and the variable must match with the code you are using. If one is a string and the other is a boolean then you will need to convert it.
For example, does ActiveDocument.Variables("plaintiff").Value return "True" or True? These are different - one is a string and one is a boolean. Can document variables hold anything other than strings?

Perhaps try to initialize the value with

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Andrew Lockton
Chrysalis Design, Melbourne Australia