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I checked your form (http://www.jotform.com/60584050597965) and I wasn't able to replicate your issue. I was able to type in all of the form fields. I was able to enter letters or numbers. I tried this with Google Chrome, Mozilla Firefox, and Internet Explorer 8.
May I know what browser are you using? Can you please try to check if the issue is happening on the another browser. Thank you.