Hello Jess,
Sorry to hear your members still can't see the Whiteboard. The problem might be on their end. Have they tried using the web app to access the channel and see if the Whiteboard tab appears?
Additionally, have you attempted copying the Whiteboard link from OneDrive and pasting it to the channel?
I attached a video demonstrating the process of creating a Whiteboard and sharing the link in a channel. It also shows how a member can access the Whiteboard using the link. I hope it helps.
https://1drv.ms/v/s!ArDAXD8EYy4khXL0qWQegvRsaot8?e=KX7rJf
Thank you for your patience.