Rae from Shopify here.
Thanks for reaching out about the error you're running into. I'm happy to lend a hand with resolving this, so you can finish adding your contact form to your site.
In some cases, errors like this can become cached on your device, which allows them to keep appearing. To help resolve this, I recommend clearing your browser cache, or using Chrome Incognito mode instead. Additionally, it's important to make sure you're using a supported browser when accessing your Shopify admin, or storefront.
Lastly, connecting to a different internet network, or resetting your current internet connection can also help fix errors like this.
Please give all of these steps a try, and keep me posted on how it goes. If this doesn't resolve things, just reply back to me here with a screenshot of the error you're seeing and I'll be happy to help take a closer look.