env_default => default_envs

platformio.ini

 #
 
 [platformio]
-src_dir     = Marlin
-build_dir   = .pioenvs
-lib_dir     = .piolib
-libdeps_dir = .piolibdeps
-boards_dir  = buildroot/share/PlatformIO/boards
-env_default = megaatmega2560
+src_dir      = Marlin
+build_dir    = .pioenvs
+lib_dir      = .piolib
+libdeps_dir  = .piolibdeps
+boards_dir   = buildroot/share/PlatformIO/boards
+default_envs = megaatmega2560
 
 [common]
 default_src_filter = +<src/*> -<src/config> -<src/HAL> +<src/HAL/shared>