Nimble is unable to connect to Quickbooks Desktop because they do not offer a REST API for Quickbooks Desktop. You may read more from Intuit here.
We are able to connect with Quickbooks Online via our PieSync integration.
You may learn more here.
Starting April 20th 2021, the Nimble PieSync integration will no longer be available for new users to sign up. Set up the integration prior to April 20th in order to be able to enjoy Nimble + PieSync through April 21, 2022.