Hi,
Thanks for getting in touch with us!
There might be some JS validation which is not letting you input value below 0. This restriction is not from WCFM. So it might be a conflict with your other plugins or theme or your own custom code snippets. To determine that, please follow this troubleshooting guide:
https://wclovers.com/knowledgebase/conflict-check/
Only the site admin would know and your users won’t be affected by this troubleshooting.
Please let us know if you find the cause.
Looking forward to helping you.
Thank You!