It seems that since the latest update of Revolut (version 9.17), the app will now display a message asking you to set a lock screen pin on your device, and won’t allow you to proceed further.
It seems this feature is not implemented in App Support, therefore making Android apps think no lock screen pin is set even with one set in the Sailfish environment. Hopefully this feature can be implemented soon (see Unable to set lock pin for Android apps requiring it), or Revolut reverts the decision to require lock screen pin.
You can always downgrade the app to version 9.16 which seems to work just fine. In Aurora store, you can use the “Manual download” feature and enter the version string 901600899.
Revolut version 9.16 seems to otherwise work fine on Sailfish. The only feature not working seems to be submission of ID documents for KYC, which fails when the app needs to verify your identity by taking a selfie. “Camera is unavailable. There might be another application using it at the same time” is the error message I am getting. Gave up and used another phone for KYC verification, after which the app can be used on Sailfish.