Few things to help you debug this issue
- Check if there is a field called “gst_category” on Sales Order Doctype or Customer Doctype
- Check if there is any custom field that is named as “gst_category”
- Go to Settings>Company> Your Company and Change Country to any country other than India temporarily and then change back to India. This will apply changes required for GST if it has been missed during the installation
- If nothing works, post a full traceback from Browser Console so that someone from the Community can help