I have faced the same problem and have tried to contact Line Customer Service. I'm from Indonesia. And here is the reply from CS:
We regret to say that Rich menu is only available in the Developer Trial account. It cannot be used in Pro accounts or Free Plan using accounts. You can see the details on our website below. https://at.line.me/id/plan
I'm quite sure that at the time I switch to Pro Plan, that page hasn't existed yet. At the time of writing, there are 4 columns (features, developer plans, free, and pro) and in Pro Plan, you can see all rows is marked with ✓, except for row "Multimedia menus" (which only available for Developer Plan)
Okay, what if we want to downgrade to Developer Plan? You have to create a new account (https://developers.line.me/en/faq/tags/plans/)
Let's hope that Multimedia Menu will be available soon, or there's a workaround for this issue.