• Conclusion: don’t install laravel/framework v5.7.9
  • cviebrock/eloquent-sluggable 4.8.x-dev requires illuminate/config ~5.8.0 -> satisfiable by illuminate/config[5.8.x-dev

>composer require cviebrock/eloquent-sluggable

Using version ~2.0 for cviebrock/eloquent-sluggable
./composer.json has been updated
Loading composer repositories with package information
Updating dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.

Solution:-

Depending on your version of Laravel, you should install a different version of the package. NOTE: As of version 6.0, the package’s version should match the Laravel version. Older versions of Laravel can use older versions of the package, although they are no longer supported or maintained.

Laravel Version	Package Version
6.0	6.0.*
5.8	4.8.*
5.7	4.6.*
5.6	4.5.*
5.5	4.3.*
5.4	4.2.*

Try:- composer require cviebrock/eloquent-sluggable:3.0.0-alpha

Try:- composer require cviebrock/eloquent-sluggable:3.0.*