It looks like the forcefield of your choice is not available, and Avogadro returns to default UFF. Despite its name universal force field, it is both an old one and results should be taken with a grain of salt (i.e., other [more recently developed] force fields may be more suitable for an optimization).
Which force field did you aim for? What version of Avogadro are you using? When you encounter this message and confirm by clicking on on “yes”, does the optimization start at all?
On a backup installation of old Avogadro 1.20 in Linux Xubuntu 18.04 LTS, the program offers me to select between UFF, Generalized Amber Force Field (gaff), MMFF94 and improved MMFF94s force field, as well as the one introduced by ghemical. I speculate these became available to Avogadro because during the program’s installation OpenBabel already was present.