Does anyone know if this is an issue with streamable or the forum
A bit of both, and a bit of neither. The easiest way to think about it is to compare the [video] tag to [img]. It takes a video file, and nothing else.
[img]https://imgur.com/a/NYDOF7B[/img] won't work because
https://imgur.com/a/NYDOF7B is a link to a web page, and not to an image file.
By comparison, something like
[video]https://v.omgomg.eu/dole.webm[/video] works fine, because it's a direct link to a video file that I'm hosting on my VPS:
It looks like Streamable only allows to generate direct file links to paid users. I can't immediately think of a service to recommend. Most video hosting sites force you to use their in-website players because that's how they get to serve you ads and/or collect your data.
Out of curiosity, why is YouTube not a workable solution for you?