The staff username field is not available in the Shopify API, unless your store is on Shopify Plus (the API is restricted).
As a work around, you can record your staff names manually in Better Reports.
a) Open Monthly sales by POS user
b) Click Record Users
c) A dialogue box shows up
d) Open the link this page. A new tab open.
e) Copy and paste the content of that page in the dialog box
f) Click Record user names
That's it! The user name field should now correctly show your staff names..