Fill a circle

How can I fill the circle at the end of the cylinder; in other words, how to put a round cap on the right side of the cylinder? (the blue in the image is the inside curved wall)

You have to close the edges.
Draw an edge on the “edge” of the circle to close it.

Do you want a flat end or a dome?

flat end

When I draw a “line” I get:
Not sure how draw an “edge”.

Like this. If your cylinder is inside a group or component, open the group or component for editing first.

got it. thx