There is a bug regarding this dependency:
me-no-dev/ESP Async WebServer@^1.2.3
I replaced that line with https://github.com/me-no-dev/ESPAsyncWebServer.git to get it to compile, but that's not a real solution. You can see the error discussed here:
me-no-dev/ESPAsyncWebServer#1147
Could you help find the right way to fix it?