Error when using composer require-dev.mikey179/vfsStream is invalid
You must have mikey179/vfsStream in your composer.json in require-dev section (as the problem is for require-dev and you are trying to do normal require).
Simply change the name of package to all lowercase characters.
Make this "mikey179/vfsStream" package name to lowercase.
"mikey179/vfsStream" replace this to "mikey179/vfsstream"
Just replace mikey179/vfsStream with mikey179/vfsstream