Here's the issue.
Your requirements could not be resolved to an installable set of packages.
Problem 1
- nikovonlas/flarum-ext-web-push v0.1.0-beta.6 requires php-http/guzzle6-adapter ^2.0 -> satisfiable by php-http/guzzle6-adapter[v2.0.0, v2.0.1].
- nikovonlas/flarum-ext-web-push v0.1.0-beta.7 requires php-http/guzzle6-adapter ^2.0 -> satisfiable by php-http/guzzle6-adapter[v2.0.0, v2.0.1].
- nikovonlas/flarum-ext-web-push v0.1.0-beta.8 requires php-http/guzzle6-adapter ^2.0 -> satisfiable by php-http/guzzle6-adapter[v2.0.0, v2.0.1].
- nikovonlas/flarum-ext-web-push v0.1.0-beta.9 requires php-http/guzzle6-adapter ^2.0 -> satisfiable by php-http/guzzle6-adapter[v2.0.0, v2.0.1].
- php-http/guzzle6-adapter v2.0.1 requires guzzlehttp/guzzle ^6.0 -> satisfiable by guzzlehttp/guzzle[6.0.0, 6.0.1, 6.0.2, 6.1.0, 6.1.1, 6.2.0, 6.2.1, 6.2.2, 6.2.3, 6.3.0, 6.3.1, 6.3.2, 6.3.3, 6.4.0, 6.4.1, 6.5.0, 6.5.1, 6.5.2, 6.5.3, 6.5.4, 6.5.5, 6.5.x-dev] but these conflict with your requirements or minimum-stability.
- php-http/guzzle6-adapter v2.0.0 requires guzzlehttp/guzzle ^6.0 -> satisfiable by guzzlehttp/guzzle[6.0.0, 6.0.1, 6.0.2, 6.1.0, 6.1.1, 6.2.0, 6.2.1, 6.2.2, 6.2.3, 6.3.0, 6.3.1, 6.3.2, 6.3.3, 6.4.0, 6.4.1, 6.5.0, 6.5.1, 6.5.2, 6.5.3, 6.5.4, 6.5.5, 6.5.x-dev] but these conflict with your requirements or minimum-stability.
- Installation request for nikovonlas/flarum-ext-web-push ^0.1.0@beta -> satisfiable by nikovonlas/flarum-ext-web-push[v0.1.0-beta.6, v0.1.0-beta.7, v0.1.0-beta.8, v0.1.0-beta.9].
Installation failed, reverting ./composer.json to its original content.