Quoted from pete_d:

It will be challenging to get a good conversion from STL to STEP. That said, not necessarily impossible, depending on the complexity of your design. See e.g. https://forums.sketchup.com/t/exporting-to-step-format-stp/5649 and https://forums.sketchup.com/t/export-to-step-file/170190. One post in there suggests that the FreeCAD program can do a good job converting STL to STEP.
If you are using a paid version of SketchUp, see e.g. https://homedesigninstitute.com/question/544/can_sketchup_export_step_files/.
A quick search shows a number of other possible solutions, e.g. https://anyconv.com/stl-to-step-converter, https://3d-convert.com/en/convert/stl-to-stp.html, https://all3dp.com/2/stl-to-step-how-to-convert-stl-files-to-step, etc. I didn't bother to actually look at those pages...I figure you can do that and see if they suit your needs.
For future reference, you will probably find other 3D design tools more appropriate, both for dealing with situations where you'll need a STEP file instead of STL, and just from a general feature and reliability point of view as well. SketchUp is really bottom-of-the-barrel in my opinion. Tinkercad is a great place for beginners, and other more full-featured programs like Fusion 360 or OnShape (free for non-commercial use), or Solidworks (a not-unreasonable $100/year), are more powerful and yet are IMHO much easier to use.
You may find it more expedient to just redo the design in one of those programs. Having done it once already, you may find the workflow a little more efficient, even if it means learning a new program at the same time.