I have a solar generator. What does it mean, "Enter Any Fuel Cell Property Improvements Fuel cell property costs main home only

For the fuel cell property costs, you will enter the amount you paid for the property (generator) only.  

If you purchased an entire solar system, the panels are entered separately under solar electric costs. 

A fuel cell converts compressed hydrogen gas to electricity.  A solar system is not a fuel cell system.  Solar systems are entered on a different line.

A solar system only qualifies if it provides power to the residence (hard-wired).   You can include the entire cost of the system including all components needed to make it work (solar panels, installation, inverter, batteries, connection to the electrical mains).  However, a portable solar generator is not eligible.