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

[Dominus77]

Your requirements could not be resolved to an installable set of packages.
 Problem 1
    - Installation request for dominus77/yii2-sweetalert2-widget 1.2.1 -> satisfiable by dominus77/yii2-sweetalert2-widget[v1.2.1].
    - dominus77/yii2-sweetalert2-widget v1.2.1 requires bower-asset/sweetalert2 >= 6 -> no matching package found.

Learn More https://asset-packagist.org/

I was helped by the reinstallation of Yii2 on the following instructions:

1. Delete folder vendor
2. Delete file composer.lock
3. Editing composer.json add section repositories, minimum-stability can be left stable

"repositories": [
        {
            "type": "composer",
            "url": "https://asset-packagist.org"
        }
 ]

https://github.com/yiisoft/yii2-app-advanced/blob/master/composer.json
https://github.com/yiisoft/yii2-app-basic/blob/master/composer.json

4. Add in config aplication

$config = [
        ...
        'aliases' => [
            '@bower' => '@vendor/bower-asset',
            '@npm'   => '@vendor/npm-asset',
        ],
        ...
    ];

5. Run commands at the root of the project

composer self-update
composer global require "fxp/composer-asset-plugin:^1.3.1" --no-plugins
composer install
composer update

Done!

These actions will delete the vendor \ bower folder.
All bower and npm packages will be placed in the vendor \ bower-asset and vendor \ npm-asset folder.