What is JotForm?
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You have 2 options to choose from:
1. Use Conditional Logic - IF its VALUE is LESS THAN 0, THEN do something (e.g. hide the submit button, show an error, etc.)
Here's how I setup the Condition:
Here's a related guide on how to setup Conditional Logic [GUIDE: Smart-Forms-Using-Conditional-Logic]
2. Use Input Mask - This will only be applicable if you know the number of digits of the field in question beforehand (e.g. your field only requires 2 digits - 23, 56, and so on). You may then set the Input Mask of the field to ## so it only accepts 2 numbers (no negative sign prefix).
Here's a related guide on how to setup Input Masks [GUIDE: Getting-Started-with-the-basics-of-Input-Masking]
This is a demo form https://jotform.com/62084737125961 showing both approaches. Feel free to clone it so you can see how the condition and input mask were setup [GUIDE: How-to-clone-an-existing-form-from-a-URL]
I highly recommend you take a peek at the links I included on this thread. Give it a whirl and if you're stuck on something, let us know we can further help.