The corners of your wall aren’t all in the same plane so you (or SketchUp) has added a diagonal line to let it bend.
I marked the four corners of the top half of the wall with the windows in it. With the precision turned up on the display of sizes. There’s two y coords which wouldn’t stop it forming a face but not ideal as it’s probably meant to be parallel to the x-axis. There’s also a variation in the x 332.272063 and 331.272063.
-332.272063", 175.961752", 288.000000"
148.727937", 174.024252", 288.000000"
148.727937", 174.024252", 150.162648"
-331.272063", 175.961752", 150.162648"
You’ve got two lines running really close to each other and not parallel up the corner of the building near the door at the top of the stair.
You should turn off “Enable Length Snapping” in Window->Model Info -> Units as this can sometimes cause problems with things not lining up.
These problems can be fiddly to repair, especially if you’re brand new to SketchUp. It’s often quicker to just redraw the wall than trying to move points a few thousands of an inch.
Here’s what happens when I start with a flat rectangle and move one of its points out of plane with the other 3: