Using Robocopy to copy files from File Server to SharePoint Online Library

I'd suggest rethinking this idea.  I don't see how robocopy is going to help for this scenario nor can I see it working, based on what's been said so far.

300 GB (if that's right?) is a large migration and the last thing you want to do is just dump it onto SharePoint if that's the plan. Usually, if this hasn't been done already, there will be a data cleansing exercise, in other words, how much of that data truly needs to be migrated to the cloud. Just as important things like Information Architecture and Governance need to be mapped out, to give a solid foundation, depending on requirements.

I'd suggest looking at a SharePoint migration tool like ShareGate, that can do all sorts of things, like map security, bring across meta information, apply a taxonomy etc.  You may come across a 5K list view limit if not careful as well, something to look out for anyway.

Have a look at this for practical steps to get started, though it's not a recent article, it also has a long list of 3rd party products that can help:

https://blogs.technet.microsoft.com/ptsblog/2013/11/04/migrating-file-shares-to-sharepoint-online/

Also, depending on how many licences you have, you may be able to get some onboarding and migration assistance from Microsoft - https://technet.microsoft.com/en-gb/library/office-365-onboarding-benefit-process.aspx.

Hope that helps a bit.