When I test the rule, Murus say there is a syntax error (but does not say what error).
You can use the TEST button in Murus toolbar, sometime it is able to tell you more about the error.
By the way Murus is a front end, not a firewall. It is not aware of errors, it can only report errors reported by PF.
Now maybe my thought is totally wrong: When I add a rule using Murus, this rule ***SHOULD*** be syntax checked .... am I wrong?
Yes in this case you are wrong, Murus is only a front end, it cannot check the syntax for PF. The very same rule string can be considered correct in some PF scenarios, and wrong (I mean at syntax level too) in some others. Murus cannot be aware of that, so you must feed PF with rules, and await for PF to react somehow, and expect Murus to catch this "reaction" and inform you.
Ideally you add a rule, then click TEST to verify if that rule sounds good to PF, then if everything is OK click PLAY to actually enable runtime rules.
In this specific case you rule lacks a source and a destination addresses, that's the reason for the error.
I hope it helped