'Sandbox Resources are not available at the moment' - ISSUE

The HTTP 412 response says the Teams validation service could not make its sandbox available; it does not show that your app package is invalid. Validate the manifest and ZIP in Developer Portal for Teams or Teams Toolkit, fixing any reported schema, icon, permission, or domain errors. If those checks pass, sign out and back in, retry after a reasonable interval, and have an administrator check Microsoft 365 service health for a related advisory. Avoid repeatedly changing a valid package or making rapid retries, because neither action provisions the missing sandbox. If the same package still returns this exact 412 in a second browser or Toolkit, capture the UTC time, tenant ID, Teams app ID, package version, validation run identifier, and complete network response. Submit those details through Teams developer support. A persistent sandbox-allocation failure needs service-side investigation; there is no supported client command to create the resource manually.