Welcome to the forum!
I'm not sure exactly what the problem is. That is a normal rectangle for Inkscape to draw. But I'll make some guesses.
Guess #1 - You don't want rounded corners. In that case, you can click this button on the Rectangle tool control bar (while the rectangle is selected)
Guess #2 - You don't want it filled with black. In that case, click whatever color you want, on the palette. Or if you don't want a fill at all, click the big X button in Object menu > Fill and Stroke > Fill tab (looks like this
)
The yellow line is called a "stroke" in Inkscape. That can be removed in the same way as the fill, except using the Stroke Paint tab of the Fill and Stroke dialog. Or you can keep it, and change the color, by holding the Shift key while you click any color on the palette (or configure some custom color using Fill and Stroke dialog). (The stroke can also be made wider or thinner, using Stroke Style tab.)
