Problem redirect no work, File not found.

Hello

i have a problem with Plesk, when i try :

DefaultLanguage fr-FR

Options -Indexes +FollowSymlinks

RewriteEngine on
RewriteBase /

<Files .htaccess>
order allow,deny
deny from all
</Files>

RewriteRule ^b.php$ /a.php [QSA]

I have this error : File not found.

I use : PHP 7.0.2 & Apache 2.4.x

Anyone can help me please