Cracking-the-Coding-Interview | Solution and analysis of the interview classics | Learning library

I am pretty new to working with jupyter notebook. I overall like it, although I sometimes get some weird errors that sometimes appear and then sometimes do not. For example, I have a data set that looks like this (showing the .head()):

Now, if I set say volume = data["avg_volume"], and then say volume.head() I get this:

But lets say I delete that line and put it somewhere else, I sometimes will get this error: