There’s currently a widespread outage for USAA. You can read a bit more here. You can check in on the status of this issue, and whether the ETA is being extended, on this dashboard: Institution Outages - Google Sheets.
Outages can cause a variety of issues, and since this is only happening with your USAA accounts it’s likely not a spreadsheet issue but just an issue with the bank connection. There’s even a forum here on the USAA Community page where customers have expressed issues with USAA’s connection to other apps (not just Tiller)