What does this mean?

I'm assuming you probably tried to open something (even though you say notifications were disabled) or the app itself was trying to open something but it couldn't because it detected that the session was locked, thus preventing it from running. SBmainworkspace (I'm assuming) is what controls/oversees applications or other processes running in the background, and automatically closes them or lowers their data usage. Since it detected that your phone was locked, it prevented TikTok (in this case, the OS sees it as musically) from opening. This wasn't from TikTok, this was a notification from your system telling you the command to run the application failed. I'm no tech expert, so please feel free to correct me on anything I got wrong on this.

It's a bug. Lots of apps trigger these SBMainWorkspace errors citing the phone being locked.

If you Google or even search here in Reddit the words "SBMainWorkspace" and "unlocked" in quotes, you'll get many results for many apps. The most common are TikTok and Snapchat.

By the way, after "unable to launch", the technical name of the app is shown. TikTok bought out Musical.ly and adopted that app's technical name "com.zhiliaoapp.musically" - that's not a website. Snapchat's technical name is "com.toyopagroup.picaboo".

If you're in the U.S., the app may continue to exhibit more and more bugs as it isn't being updated at this time due to it being removed by Apple and Android.