MKS GEN L unknown motherboard error
The MKS GEN L board was added in Marlin release 1.1.7 .

According to the configuration file you've linked and previous Marlin releases, your copy of Marlin is release 1.1.6. While all Marlin releases before 1.1.7 had "010100" as "CONFIGURATION_H_VERSION", earlier versions had less features than listed in your configuration file.

In Marlin 2.0.x, the MKS GEN L is also available.

Since you said it was also unavailable in 2.0.x, please provide a copy of your entire 2.0.x setup.