For me this happen because the incompatibility of each package used.
I think the packages that relate to build runner are floor_generator
and retrofit_generator. You can simply resolve the issue by following these:
remove retrofit_generator, floor_generator and build_runner from dev_dependencies
add packages from terminal ->
flutter pub add dev:build_runner
flutter pub add dev:floor_generator
flutter pub add dev:retrofit_generator
try to build the build_runner again ->
dart run build_runner build
or if the error still occur try this package version:
dependencies: floor: ^1.4.2 retrofit: ^3.0.1
dev_dependencies: build_runner: ^2.4.6 retrofit_generator: ^4.2.0 floor_generator: ^1.4.2