The staff username field is not available in the Shopify API, unless your store is on Shopify Plus (the API is restricted).
If your store is on the Shopify Plus plan, you can skip this page and record your staff names automatically instead. See User/staff reports.
As a workaround, you can record your staff names manually in Better Reports.
a) Open Monthly sales by POS user or any report which has user name.
b) Click Record a snapshot of staff data in the dialogue box that shows up at the top of the report.
c) A dialogue box shows up
d) Open the link of this page. A new tab will open.
e) Copy and paste the contents 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.