When installing Magento 2 extensions you can see 404 Error Page Not Found, as Magento 2 Admin Panel (backend) is not working. 

Magento 2 admin not found

Issue #1: the result of installing or updating

You can see the ‘error’ message after:

• installing a new extension;
• upgrading extension;
• applying Magento 2 core or extensions patch.

Solution #1: flush Magento 2 cache

via the Command line

Type in the next command to your Command line:

php bin/magento cache:clean
rm -rf var/cache/*
rm -rf var/generation/*
rm -rf generated/* (for magento 2.2.x)

Issue: the compilation hasn’t been restarted

When installing a new extension in production mode you may forget to restart the compilation. As a result neither the backend nor the frontend work.

Solution: reset the production mode again via the Command line

• Use the command to check if the production mode is activated:

php bin/magento deploy:mode:show

•  If the currently applied mode is production, run the command:

php bin/magento deploy:mode:set production
to set the mode once again and reset everything from the start.

Issue: an incorrect admin URL

In some cases admin users can apply an incorrect URL to the Admin panel, for instance, you can use https://magento.com/backend instead of https://magento.com/admin.

Solution: learn the admin URL via the Command line

Run the next command to learn a correct URL leading to the Admin Panel:
php bin/magento info:adminuri

 Learn more about Magento 2 Admin URL change.

 Issue #2: the permission role is set incorrectly

Due to this issue, you may receive the following error in the Magento 2 backend:

• ‘404 Error’;
• ‘Page not found’.

Solution #2: reset the permission role of your administrator

Go to the Admin Panel> Systems> Permission> User Roles:

• Select the role name that needs to be edited, e.g.: ‘Administrators’;
• Switch to the ‘Role Resources’ menu:

• Set the ‘Resource Access’ to ‘All’;
• Save the role and clear Magento 2 cache after the settings are completed.

Issue #3: just-out installation on Ubuntu, CentOS, Debian…

In this case the ‘Not Found’ message is displayed on the frontend when you try to access a page.

Solution #3: edit the configuration file

E.g.: Ubuntu file /etc/apache2/apache2.conf

• use the command:

sudo vi /etc/apache2/apache2.conf

• change the piece of code:

<Directory /var/www/>
Options Indexes FollowSymLinks
AllowOverride None
Require all granted
</Directory>

• for this one:

<Directory /var/www/>
Options Indexes FollowSymLinks
AllowOverride All
Require all granted
</Directory>

• use the command to restart Apache:

sudo service apache2 restart.