KiCad 6

klepcbgen generates files in kicad 5 format. Fortunately, kicad 6 is able to read these files and save them in the new format. Unfortunately however, some of the footprints changed between kicad 5 and 6 which results in a number of errors when updating the footprints. For now you'll have to change these footprints manually. In particular, you need to change the footprints for the SMD capacitors and resistors.

To do this, perform the following steps:

1. Hover on the outline of one of the capacitors (and press "e" to open the properties (if you hover on e.g. a pad it'll open the wrong dialog)
2. Click "Change Footprint..."
3. Select the option "Change footprints with library id:" (leave the id unchanged)
4. Change the "New footprint library id" to Capacitor_SMD:C_0805_2012Metric_Pad1.18x1.45mm_HandSolder
5. Click "Change"

Repeat these steps for the resistors, changing their footprint library id to Resistor_SMD:R_0805_2012Metric_Pad1.20x1.40mm_HandSolder

Note that the "Match Method" mentioned in the usage section is called "Re-link footprints to schematic symbols based on their reference designators" in kicad 6.