if you are creating a new App using the hub you must attend to the the server is not your computer. When you try to accés to a path, this is on the server not in your computer.
Try to copy the files you want to load into the server directory and create a new data connection to this new folder.
As others have mentioned, C:\Users\...\ needs to exist physically on the server that is running Qlik Sense (and more technically any node with the Engine service). For example here from one of my VMs:
I want to also say that this is a poor choice from a governance standpoint. Rather than having data files living in folks' users directories, it's far more ideal to place them in a central location, e.g. C:\Data\ so you can have 1 data connection connecting to multiple data files.