cant add github heroku buildpacks

Hello! Im new to, i have an application that makes use of the node-canvas module, and as known, you need to use a custom buildpack to use Cairo based dependencies. the problem is that it cannot find a tar entry. any idea of how to add them, thanks.

Have you tried without a buildpack (in other words, using fly launch to provide a Dockerfile)? I just tried adding canvas to an existing application and it deployed successfully.