Heroku app not launching : process exited with status 0
This part of the log file looks suspicious
Starting process with command `: python3 test_discord.py` ^^The colon shouldn't be part of the command.
The reason for that colon is your Procfile, it has a surplus space:
worker : python3 testdiscord.py ^It needs to be:
worker: python3 testdiscord.pyThe exit code is 0 because a colon in shell does nothing and always returns with status 0.