Basically hardwood lumber is a raw material so it will have some flaws. Not a lot, but some.
What this means to you is sometimes you can get nearly clear boards, others will have parts you don't like, and it's just a part of woodworking.
We stock the highest grade of lumber that we can buy, which most of the time is no less than 83% clear on one side. Most of the time it's better than that.
But, sadly, knot-free wood can't be guaranteed 100% of the time.
In hardwood lumber, believe it or not, there's no official "clear" grade. Lumber grades are determined by the National Hardwood Lumber Association, not by us, and the highest grade is called FAS. The rules for that grade mean the boards have to be 83% or more clear on one side. More about lumber grades here >
Fortunately, in most projects you're cutting your parts from boards and arranging them for color and grain, plus usually only one side of a board shows in a finished project.
When you come upon a knot, flip the board over to see what's on the other side. You might be able to work around it that way.
Otherwise, plan for a waste factor to give you enough wood to arrange your boards exactly how you want.