It’s perfecltly fine, works the same was as “Je peux t’en donner la raison”.
Nowadays people might ommit the en and just say “Je peux te dire la raison”, but I’d be willing to bet it’s more gramatically correct to use en, and at least to me it sounds better
If you search for inurl:"https://fr.wikisource.org/wiki/" "en dire la raison" on google you’ll find plenty of examples from classical texts