HOANGVANHIEP as luceos said, when following the composer install steps after using Terminal to upload flarum/flarum to your site, it will walk you through all steps. Again, if you followed all steps and you received that error then it may be a discussion to have with your webhost.
Also after uploading flarum you can continue the install steps from your browser at your install direction in the URL bar. Sorry if that sounds confusing... long day at work.