Wrong statement, you need to close your anonymous ACL with }, instead you have that at the end.
Use:
acl url_staging path_beg /123456
use_backend staging if { ssl_fc_sni staging.com } AND url_staging
acl url_production path_beg /789012
use_backend production if { ssl_fc_sni production.com } AND url_production
And since AND is implicit, you can just:
acl url_staging path_beg /123456
use_backend staging if { ssl_fc_sni staging.com } url_staging
acl url_production path_beg /789012
use_backend production if { ssl_fc_sni production.com } url_production