STL file not found

I have Octoprint 1.5.2 and an Ender 3 Creality 1.1.6 (purchased used)

This is a fresh install of Octoprint and Octopi. I have rebooted already both devices.

I'm getting this error when I try to print an STL file

Upload failed
Could not upload the file. Make sure that it is a readable, valid file with one of these extensions: .g, .gco, .gcode

I've tried to Google the error but I'm running into a lack of solutions. I'm not too tech savy.

What am I missing? I thought STL files were supported for printing.

Printers work with gcode - a set of instructions for how the printer should move - STLs are model files. To convert your STL into a gcode file you need a slicer. Popular slicers include Cura and PrusaSlicer. You will need to use a slicer first to generate your print, then you can upload it to OctoPrint.

Thank you for the info, I appreciate it very much!