When ever you need to let Rex access the Xero integration, you need to give Rex authorisation. Because of the security levels when dealing with Xero, you'll need to authorise the integration while using it. This will only appear when needed.
Simply:
- In the new Authorise popup, click Xero | Authorise Application.
- Enter your login details for Xero.
- Click Login - once you've done this, go back into the Rex window and continue.
Rex will save Xero credentials for up to 60 days, however if you switch between multiple Xero accounts, you'll need to disconnect Rex from Xero before logging into another account.
This can be done within Xero, under 'Organisation Settings' > 'Connected Apps'.
The below screenshot shows you where to find this;