Facebook has instituted a new policy for Pixel in which your domain needs to be verified in order to continue using it. While the HTML cannot be updated on your Podia site, you can add the DNS TXT record to verify this if you have a custom domain name!
The article below links to this verification method:
What if I don't have a custom domain name setup for Podia?
At this time, you will need to setup a custom domain in order to continue using Pixel, as we cannot modify the DNS records for your Podia subdomain 💜