Hello,
l2_e1 and l2_e1_sol are generated and built no error.
They flash, but they don't load the program the nRF52 DK.
I used several versions of the toolchain and SDK.
I selected the optimize for debugging option and without the option selected.
I flashed other programs from the nRF Connect SDK Fundamentals course and they all load normally and work.
regards
Luiz