why an I not see the point of circle command on my context menu
Which version of SketchUp are you using?
In SU 2016 there’s a built-in command Find Centre. Select a circle or arc, Right click, and choose Find Centre
That command puts a guide point at the center, which may be what you want. An alternative if you have a drawing tool active is to hover the cursor over a vertex of the circle’s representation and then move the cursor to the approximate center. The first action causes SketchUp’s inference engine to conclude that you are interested in the circle as such, so it provides a snap at the center when you next move the cursor near it.