How do I fix a purchase event error with my FB pixel?

My FB pixel is reporting an error with the purchase event and giving this message:-

“Open your website’s source code. Locate the Purchase event(s) and update the event code snippet. The value field must contain a decimal number greater than zero, and may not include letters, special characters, currency symbols or commas”

Can anyone tell me how I go about doing this? Where do I find the purchase event and what does it look like and how do I change the value - ie to what?

Theme is Motion

Many thanks