In order to map a field you need to make sure that the field type matches for the fields that you're trying to map. This means that you can only map a Numeric field to a Numeric field and a Text field to a Text field etc.
For example, a Text field could ONLY be mapped to a "Single Line Text" element/field on your web form. Similarly, a Large Text field type can ONLY be mapped to a "Paragraph Text" element/field.
Our system will automatically hide non-matching fields from the "Map Rules" dropdown. So for example, if you're mapping a Date field, it would only display those fields in the list that have field type set as Date: