@Ziggy Zulueta Thanks for reaching out to Microsoft Q&A, apologize for any inconvenience caused on this.
Based on the shared information, I understand that you want to downgrade the static webapp tier from standard to Free. As you have enabled Enterprise-grade edge it is failing with above error.
Could you please disable the enterprise-grade edge using the below cmdlet and retry scaling down to Free tier.
az staticwebapp enterprise-edge disable --name --resource-group
Hope this helps, let me know if you have any further questions on this.