So this is a standard player misunderstanding.
Ranger learned speak with animals, wanted to use it as flavour and to aid in the befriending of a beast, (giant poisonous snake). When it came to the roll, instead of using the rangers animal handling (+6) the spell actually made the task more difficult with persuasion being a (+0).
Question 1. Does this make sense rules as written?
Question 2. Does this make sense period?
My take? If a you have a pet dog, trained from a pup that you know and communicate with well already (think +6 animal handling with this particular dog) and you suddenly had the ability to speak perfectly to it and it back to you, would it make it more difficult to communicate with the dog? In my mind, hell no. In 5e rules as written D&D maybe?