Can't get extrude working correctly
I am using Creo Parametric 10.0.2.0
Background: I am an experienced Electrical Engineer, have used AutoCAD my whole career, mostly 2-D but have done 3-D objects. I recently learned Altium and have done 2-D and 3-D modeling there. I have used Creo to view objects in the past as well as creating assemblies with pre-existing 3-D models. In short, I have background in CAD and 3-D modeling, and in Creo, but not in Creo 3-D modeling from scratch. However, today I have been asked for the first time to create a 3-D model in Creo - and I am struggling.
I am starting with the simplest model imaginable, but I have yet to grasp it. I just need a flat panel. The panel is a flat rectangle with small rectangular "ears" with a thickness of 1.5 mm.. I created a sketch of 3 rectangles: one large and two small rectangles each on the center of the long side. Should be easy, I should just need to extrude it with a dimension of 1.5 mm at this point.
Obviously, extrude from surface is the wrong choice - I'm only mentioning it here, because it "works" and does what I expect. However, it of course just extrudes the lines of my sketch, so I get three hollow rectangles with a 1.5 mm thick frame around it. That is not what I need, I need this to be a solid.
If I try to extrude from solid, one of three things happen. Sometimes, It won't even let me select my sketch. Other times, my sketch is selected, but it won't let me use a thickness of less than 590 mm. And lastly, sometimes it lets me put in my 1.5 mm thickness, but then the whole thing becomes invisible.
My next thought is well, my sketch isn't solid, it's just the outlines, so I need to make it a solid plane. However, nothing I have tried works. Fill won't let me select it, and solidify is greyed out,
I'm sure I'm missing something fundamental and easy here, but I've watched multiple videos and I haven't figured out what I'm doing different / wrong so I appreciate any help that someone can offer.

