How do I suppress the warning "unable to find the application log" after running "symfony serve"?

I have bumped into the same issue today. For some reason, I had to create an empty log file (dev.log) inside the var/log directory. After that, the warning disappeared and the log messages appeared in the file.

You have to install the "logger" package:

composer require logger

I had the same problem. The composer update command in the terminal automatically built all missing files and in particular var/log/dev.log.

This might happen when you've altered the default log file path. In my case, I use rotating_file to make sure dev.log doesn't grow too large. rotating_file writes to dev-yyyy-mm-dd.log instead of dev.log. Creating an empty dev.log removes the message.