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I checked your form and everything looks to be loading without issue. I was able to see/use the signature panel and the submit button in your form:
Is your form embedded to a web page by any chance? The only thing I can think of is that your form is embedded to a webpage and something may be conflicting with it. If this is the case, try replacing your embed code with the iFrame code:
If that is not the case, if possible, let us know the web browser your client was using when they were unable to complete the form. As soon as we can reproduce the issue, we can work on getting it sorted.