File-system doesn't work on github

Hello, so i’m coding a discord bot, and I’m using ‘file-system’, so when I write a specific command, the bot will write in a file my discord id, I tried locally (the bot online on my pc) and that worked, but when I tried with github and heroku that doesn’t worked

here’s my code :

 fs.appendFileSync('./ami.txt', ` \n`);
            fs.appendFileSync('./ami.txt', `${msg.author.username}#${msg.author.discriminator}\n`);
            fs.appendFileSync('./ami.txt', `${msg.author.id}\n`);

Pls help ;D