Newbie in Magento 2 and need help

[roman_scholz]

Hi,

at first, I hope it's the right forum to my issue. I got a task to have work with Magento 2 and I'm totally new in it.

I'm using Windows 10 and Xampp with PHP 7.2.4. PHP 7.2.4 is the reason, why I have to use Magento 2.3-develop from GitHub. I had no issues with the Installation and currently I try to install the sample data from GitHub.

That's my source for the steps.
https://devdocs.magento.com/guides/v2.3/install-gde/install/sample-data-after-clone.html

So, I never had before in any framework, who I'm working with in Xampp, to change file system ownership and permissions. Actually I thought it would be chown -R :apache . or maybe chown -R :xampp ., but I only get this one. chown: invalid group:.

Maybe someone could explain to me, how I can resolve the next step, to get install the sample data.

[stephaniie]

The DOC sucks for Magento. 

Yes. This might seems like a very easy thing to do. Install a Theme.

However, Magento for newcomers is really tricky. Is the installation process difficult to benefit the extreme high price for installation from Theme sites ? 180$ to install a Theme. Seems like a robbery. Maybe it even is.

Anyhow,

My mission is to install one theme. I downloaded a free one and now I'm trying to install it using Xampp.
All help is appreciated.

My first try:
Pre-requisites:
Installed Xampp with php 5.6
Installed Composer.

Step 1: Clone Magento2 from github to htdocs/ folder.
In command prompt I copied the branch 2.1 of Magento2 by writing:
git clone -b 2.1 https://github.com/magento/magento2.git

Step 2: Install Theme's database
A ) create a password for PhpMyAdmin . ( https://www.coderhold.com/how-to-change-or-set-phpmyadmin-password-on-xampp.html )
B ) Create a database in PhpMyAdmin and call it magento_db
C ) Copied the database.sql from theme.zip to /xampp/mysql/bin/
then I open cmd from bin folder and wrote: mysql -u root -p magento_db < themedatabase.sql
( Works better on larger sql files )

Step 3: Install Theme .
Add the quickstart.zip file to the htdocs/magento2/ folder.
Unzip and press "Yes" to overwrite files. Wait for process to finish.

Step 4: Install Magento2
Open localhost/magento2/ in URL . And follow install process.

But I get this error.

Autoload error
Module 'Magento_AdminNotification' from 'D:\xampp\htdocs\magento2\app\code\Magento\AdminNotification' has been already defined in 'D:/xampp/htdocs/magento2/vendor/magento/module-admin-notification'.

Help please?