I don't think I've heard of HubSpot not allowing vertical videos within the social tool. I'm also not seeing any mention of that limitation within the social post HubSpot Knowledge Base article.
It looks like you have a Starter account based on your post tag, so it's worth noting that the social tool is accessible on with a Marketing Professional subscription or above. I'm wondering if you're looking in a different place if you don't have access to the social tool?
I wasn't sure what level of tool I have because I have an account under a client. Is this information under my Profile? Either way, I am using the HS social tool for scheduling so I must have Marketing Professional.
I'll drop the error message I receive in the comments of this record. Thank you.