Yes, use pip (or pip3) to install sympy  It should install the dependencies too.   Don't use the dnf packages for what you want to do.  I know it is confusing.

I am NOT a Python or pip expert.  But I've run into similar errors and have always tracked it down to something not being installed properly.  I'm not sure which version of sympy that Octave is expecting - Python(2) or Python3.  I've been tripped up before by having the wrong version of a package installed.   

Nothing against the excellent Fedora people here, but you might get more help on https://www.reddit.com/r/octave/.