404 not found error for admin page

For Linux edit the file /etc/apache2/apache2.conf

To edit this file use sudo vi /etc/apache2/apache2.conf command

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

to

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

After this run command

sudo service apache2 restart

IF STILL ADMIN SHOW 404 NOT FOUND, FOLLOW THIS STEP

1) Add .htaccess at root of magento 2

https://github.com/magento/magento2/blob/2.3-develop/.htaccess

2) Now run this command

php bin/magento setup:upgrade

php bin/magento setup:static-content:deploy

rm -rf var/cache/*

rm -rf var/generation/*

Please try Below steps

1. Delete var/cache folder
2. Go to database SELECT * FROM core_config_data WHERE path = 'web/seo/use_rewrites' and make it 0
3. Then enter url in your browser magento_base_url/index.php/your_admin_url

try and let me know if you face any issue.

I also had this issue, it turned out one of my URLs was set to the IP address and not to the domain name so going to the {domain}/admin was returning not found.

Solution was to run

php bin/magento config:set web/secure/base_url https://domain.com/
php bin/magento config:set web/unsecure/base_url https://domain.com/

Don't forget the trailing slash, it will not work without it https://alanstorm.com/invalid-base-url-value-must-be-a-url-or-one-of/