>>...Is that good? Bad?
Whether that's good or bad depends, but at the end of the day the important thing is that you should communicate the status fully to the person who assigned you the task, rather than obfuscate it by saying things like "still working on it" or "it's a bit trickier than I thought".
Being assigned tasks you don't know how to do is fine. It's how you grow as a professional. However, there is a fine line between a task you don't know how to do but can learn in a reasonable amount of time, vs. a task you don't know how to do and can't learn in a reasonable amount of time. The latter, in my experience, is what results in the "Canary MacGuffin" scenarios Rachel describes, and it can be frustrating for everyone involved.
Ultimately, the responsibility for figuring out the difference lies with the team lead or scrum master or whoever assigns tasks in your company. But they need you do be transparent with your level of skill and knowledge to be able to do that.