Backup your .htaccess file, if any is existing. Make sure you save it somewhere. Every site has a different need, so if it's not a fresh site, someone might have included essential configuration there already.
The .htaccess file is probably not configured with the default WP code.
Create a new file in file manager > .htaccess > with the following:
# BEGIN WordPress
RewriteEngine On
RewriteRule .* - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}]
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
# END WordPress
After creating, load the browser again and make sure the .htaccess is there. Open the file and check if it's not empty. Past the code from the link above inside the file and click save.
You might have to install the classic editor, also. To use blocks though, go to WP > Settings > Writing > choose blocks.