python3-certbot-nginx is only available on Debian based systems.

see #437

modoboa_installer/ssl.py

@@ -92,13 +92,14 @@ class LetsEncryptCertificate(CertificateBackend):
         else:
             utils.printcolor("Failed to install certbot, aborting.", utils.RED)
             sys.exit(1)
-        #Nginx plugin certbot
-        if (self.config.has_option("nginx", "enabled") and
-            self.config.getboolean("nginx", "enabled")):
-            package.backend.update()
+        # Nginx plugin certbot
+        if (
+                self.config.has_option("nginx", "enabled") and
+                self.config.getboolean("nginx", "enabled")
+        ):
+            if name == "ubuntu" or name.startswith("debian"):
                 package.backend.install("python3-certbot-nginx")
 
-
     def generate_cert(self):
         """Create a certificate."""
         utils.printcolor(