It looks like something is trying to require('server') somewhere in your app, which isn't necessary. The error is coming from module. Js which deals with require-ing node modules.
If you could provide your Procfile and maybe some source code it might be easier to pin down the problem, but based on first glance it looks like there's a relative path pointing to /app/server. Js somewhere in your server code which shouldn't be doing so.
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