The issue has been resolved. The government had changed the URL for checking an EPC.

We have received reports that the EPC feature using the certificate number is failing in Tenancy Builder. We are investigating the issue.

As a temporary workaround, please follow these steps:

1. Go to the portal to find EPC at https://www.gov.uk/find-energy-certificate
2. Search by postcode or certificate number
3. When viewing the EPC, click print and save it as PDF
4. Return to Tenancy Builder, and for the EPC, choose "upload" and upload the PDF just created.

We apologise for the trouble, and we are looking into the issue.