Plugin Installation Error in Craft 3 / PHP Version
Usually when you see that error, Craft was composer install/update'd on one environment with say, PHP 7.2.  Composer then pulls in the highest supported dependencies for that environment.

The composer.lock file is pushed to a different environment with say, PHP 7.1, creating a conflict the next time Composer needs to run.

You can do this to force Composer to use the lowest common denominator for all of your environments: https://github.com/craftcms/craft/blob/master/composer.json#L34-L36