What does it mean for data on a scatter plot to have no association? how does that affect the line of best fit?

If there isn't any association, or CORRELATION, then you'd have a very difficult time determining a line of best fit since the independent and dependent variables wouldn't have any form of relationship.

For instance, let's say that I was going to plot (x,y) where x represented somebody's first name, and y represented how old they were

Does a person's name determine their age?  Of course not, so there would be no association, or CORRELATION, which means there would not be a way to put in a line of best fit for this.

Hope this helps.

Brian