With Arial:

\definefontfamily[mainface][rm][Arial]

\setupbodyfont[mainface,10pt]

with Optima:

\definefontfamily[mainface][rm][Optima]

\setupbodyfont[mainface,10pt]

It doesn’t matter if the font statements are before or after \starttext

Then I thought, let’s test some other things. I logged in as another user, and used ConTeXt from TeX Live 2019:

It seems to be that on my system, the standalone installed ConTeXt does not work with Optima, but the TeX Live installed one does.