I cannot create a solid using arcs

I’m trying to make a solid to print.
Why when I use arcs tool to make round corners and use the tool to follow me it always leaves holes? preventing the creating of a solid.

Thank you

You’re surely running into the small faces issue. Work at a larger scale like maybe 100 or 1000x.

For 3D printing, set units to meters but model as if they are millimeters or inches. Export your STL file and then, when you import it into the slicer software, tell the slicer it’s millimeters or inches as appropriate.

Additionally you’ll want to make sure you only have white front faces exposed.

From the picture, it looks like the OP might not have given the walls any thickness. That’s necessary for successful 3D printing.

This is true. Since it isn’t possible to tell from the OP’s screen shot what they’ve done, I opted to give them the benefit.