This is in the Cura X-Ray view and your model is on the right. The red area is an error in the model. Go to the Cura Marketplace and load the Mesh Tools Plugin. It runs a check on models when you open them and would have told you that the model has problems. The model on the left was repaired in MS 3D Builder.
I had to rotate the model on the Cura build plate to get it to slice. That's a known bug in 5.2.1. But repaired model sliced at 200% and 300% as well.
It sounds like you opened a gcode file instead of a model file. If the Layer Slider works without having sliced it then it's a gcode file. You can also open that "bad" file in Notepad and see if it is just a mis-named Gcode file.
Try another STL and see if it will slice.
I just installed Ultimaker Cura on a new laptop, selected my printer, Ender 5 Pro, and added my chosen STL. Before I could go any further this popped up on the screen "Make sure the g-code is suitable for your printer configuration before sending the file to it. The g-code representation may not be accurate.". I checked to make sure it was not a previously sliced item... Instead of getting the option to slice the item, I only get the option to save it to a destination. Can anybody point me in the right direction?