How do I set Developer Mode in Magento 2

you can enable it from the cli

bin/magento deploy:mode:set developer

Another way, we can enable developer mode

• Goto app/etc/ and open env.php
• and change current application mode by MAGE_MODE' => 'default' to MAGE_MODE' => 'developer'

Then,please flush the cache

As you and others have noted:

SetEnv MAGE_MODE "developer"

in your .htaccess file should enable developer mode assuming that AllowOverride All is in the proper <Directory> directive.

Related to your original problem though, it might be due to a permissions issue where the Apache process user does not have write permissions to the right directories.

For a quick test, you can do the following inside your Magento 2 directory:

chmod -R a+wX var
chmod -R a+wX app/etc
chmod -R a+wX pub

Then if the installation wizard pops up, you can try to make sure you have the correct permissions for your Apache process on those directories.