Converted stp files are hard to deal with and frequently result in problems. I've never had an online converter actually work.
I use Nates script, here:
Any good way to convert an .stl file to something Alibre can process?
www.alibre.com
It's not 100%, but works better than anything i've found online.
If you don't want to try installing the command line program and script from the link above, change the file extension of the .stl file to .txt, and upload it here. I'll have a go at it with the script.
here is the command line converter tool:
Convert stl files to STEP brep files. Contribute to slugdev/stltostp development by creating an account on GitHub.
github.com
and the script is attached.