Robertos make sure both the webserver and command line PHP versions are the same and that they match Flarum requirements.
Usually, web hosts offer a setting to switch the version.
Alternatively, you could run Composer on the command line by explicitly specifying the version. For example if the web server runs PHP 7.2 but command line is PHP 7.3, you can usually run PHP 7.2 via php7.2 composer instead of composer.
If that was indeed the issue, you should re-install the composer dependencies by using the same PHP version you will be using on the web. If that's not too cumbersome, you can start the installation again and change the composer commands as instructed above. Alternatively you can delete vendor, composer.lock and run php<version> composer install in the Flarum directory to re-create those files with the correct version.
