You don’t enable the panel in MATE Tweak, it’s already on your desktop, that’s the bar(s) with applets, the clock and so on. What you can do in MATE Tweak is enable or disable the indicators, which is an alternative made for Ubuntu a few years ago to replace the system tray (the status icons like volume control, network status…). Note that with MATE Tweak, you can select various desktop layouts (e.g. one with one panel on top and one on bottom, one that copies the layout of Ubuntu’s Unity… some use the indicators, some don’t).
In other words, on Ubuntu MATE, you have two possibilities for status icons: either you use the systray or you use the indicators.
If you use the indicators and the volume control is missing, it could be that the related package has been uninstalled for some reason (indicator-sound −basically, each indicator is installed through its own package and the panel displays all those that are installed). If you use the classic systray, it’s probable that the control applet isn’t launched.
Currently, you’ve fixed the issue by running the volume control applet at system startup. But it would be a good idea to check if you use indicators overall, as maybe you currently have a mix of indicators + the volume control in the systray where you sould have an indicator for the latter.