@Jonathan-Kelly-0 Do note that the warning is there for a reason: due to differences in both the map itself and the way routes are calculated between maps, a route planned using OpenStreetMap can differ from a route calculated using HERE, which is what MRA Navigation uses. So by turning this off, you run the risk of your route changing from what you had planned.
"Usually", the differences aren't major (obviously all map suppliers want to have maps that reflect the real world and the "optimal" way to calculate a route should be similar between maps), but if, for example, the HERE map is missing a "crucial" bit of road, it can lead so a more significant deviation.
It is advisable to plan your routes using the map and settings that most closely reflect the navigation app or device that you'll be using to drive it. If you're planning a route for multiple people using different apps and devices, the MRA planner has "Compare route with" functionality to compare routes planned on one map to the other maps.