Level 18
I think I solved it. just by changing location as follow
location / {
if (!-e $request_filename){
rewrite ^(/)?api/.*$ /api/index.php;
}
if (!-e $request_filename){
rewrite ^(/)?customer/.*$ /customer/index.php;
}
if (!-e $request_filename){
rewrite ^(/)?backend/.*$ /backend/index.php;
}
if (!-e $request_filename){
rewrite ^(.*)$ /index.php;
}
try_files $uri $uri/ /index.php?$query_string;
}