Why doesn't composer install packages?

Question

Yii

Hello, I'm trying to install various extensions from katrik, for example: "kartik-v/yii2-mpdf": "dev-master", but composer constantly gives an error. In this case:

Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Installation request for kartik-v/yii2-mpdf dev-master -> satisfiable by kartik-v/yii2-mpdf[dev-master].
    - Can only install one of: myclabs/deep-copy[1.7.0, 1.6.1].
    - Can only install one of: myclabs/deep-copy[1.7.0, 1.6.1].
    - Can only install one of: myclabs/deep-copy[1.7.0, 1.6.1].
    - Conclusion: install myclabs/deep-copy 1.7.0
    - Installation request for myclabs/deep-copy (locked at 1.6.1) -> satisfiable by myclabs/deep-copy[1.6.1].


Installation failed, reverting ./composer.json to its original content.

What is the reason?