Probably, it's the path.
SQL runs as an engine that you communicate with - and that engine does not run under your user, it has its own user and its own permissions.
So SQL can't access your files - "Users\will.sewell" is not accessible to the database engine at all.
Even if it was, that would only work for local SQL Server instances - remote ones can't even access your HDD, much less any folder on it! And local SQL instances are only there for development, so you don't mess with the production server!
So either create a folder on the server that can be accessed by all users (though that's a security risk all of its own and you will have to be careful about GDPR if you go that route) or better don't use an SP at all so that the location is actually flexible depending on where the installation is.
If you're trying to backup a local DB for development because your code is likely to trash it in a minute, then try this: Backing up an SQL Database in C#
It's the way I do it!