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simply searching for fundamentals or constraints at that website would likely yield you an answer that would pay dividends down the road. if you really want to shortcut it and the the blue dot is a real hindrance…. just select everything and click on the fix constraint. 90% of the time this questions comes up its related to Scotty's comment, you haven't told the program where you want the sketch located in relation to existing geometry.

so when you define the length, width and radii of your geometry, the shape is fully defined. but the position is not!
the lower line is black and so it's probably constrained vertically, but the end vertices are blue so they can probably move horizontally still.

so you may be one (horizontal) dimension/constraint away from fully defining your sketch…

'Anchor' your sketch by intersecting a point in it to the origin or by dimensioning a point in the sketch to the origin. In the future, consider starting the first element in a sketch at the origin. - Scotty

Export to STL and set the infill with your slicer software.. youtube can help you sort out slicer settings

The infill is usually added in the slicer. I have Prusa printers and use Prusa Slicer. The Prusa Help site has tons of info on slicing, settings, etc.

Most start with Cura it seems. I'm moving to Bambu Studio

Check out Attractor Pattern. by @EvanReese

@andy_miller859 Attractor Pattern is what I usually reach for (since I wrote it), but the Texture feature by @MichaelPascoe is also good at this kind of thing.

However a lot depends on the kind of texture you want. If it's geometric, and fairly coarse (like a hexagon pattern), these custom features are good. If you want something organic, like leather, or super fine, like tiny bumps, then Blender, zBrush, or some other mesh-centric modeler will be better.