As Dan indicates, if you are building a dual endstop version of the LowRider, then you want the DualLR version of the firmware. Due to the size and complexity of the Marlin builds these days, getting the project to compile in the Arduino IDE is problematic. Instead the recommended pathway is to use Platform IO, and is documented here. If you want to try using the Arduino IDE, then the file you are looking for is Marlin/Marlin/Marlin.ino
There is an alternate, no-compile way to upload the firmware that I explain in this post. Since making that post, the binary file has been moved to the top (inside first ZIP file) and is called firmeware.hex.