Are there variable names that are illegal or you cannot use?

Yes, you cannot use the word “document” as a variable name:

Usually, you will get this error message. However, there have been instances where this variable name was able to be used in a production project. It is not known how this error was avoided. When this variable name was used in production it caused data not to be saved to the survey.

If you use another variable name and get this error, please submit a ticket and let us know. We will add it to this FAQ.