With New Teams, I think this is by design. It's to get the scroll bar out of the way when not in use. You can use the mouse wheel to scroll up and down then grab the bar with the pointer if needed. Alternatively, if you don't have a mouse wheel. You can click the Maximize/Restore Down with the little square button just left of the X close button on the top right. This resizing of the window will make the bar show up temporarily to grab.
You can see from this post Switch to the new Microsoft Teams - Microsoft Support the scroll bar is hidden when not in use.
Someone mentioned this behavior has been updated in a preview release but until then Classic Teams may be the only option.