If you want to use the concurrency support through SwiftPM, you'll need to use
swift run -Xswiftc -Xfrontend -Xswiftc -enable-experimental-concurrency
The meaning of -Xswiftc is "SwiftPM, please hand the following argument through to the Swift compiler without interpreting it". And similarly, -Xfrontend asks the Swift compiler to hand the next argument into the compiler's frontend.
Oh, and to actually run the binaries produces with the experimental concurrency support, you'll need a
export DYLD_LIBRARY_PATH=/Library/Developer/Toolchains/swift-latest.xctoolchain/usr/lib/swift/macosx/
(adapt the path to your Swift toolchain as necessary).