Have non-curators ever been allowed to add scientific name synonyms? I don’t think so, and I’m not sure they should have that ability. (Adding the ability would be a new feature request.)
Looks like you selected one of these in the dropdown, which are duplicate added lexicon/languages. These are essentially acting as normal common names, which non-curators add, and not scientific names, which only curators can add.
The true selection for adding a scientific name is at the top of the list:
When I log into a non-curator test account, change my language to Spanish, and try to add a synonym selecting that top one, I am unable to:
But if I select one near the middle of the list, I am (because it’s just a normal “language”).
This is more obvious when you change your site language, but you can see in Spanish that the “c” in científicos is lowercase for the “true” scientific name lexicon:
https://www.inaturalist.org/taxa/556727.json
See also open issue to clean up duplicate lexicons:
https://forum.inaturalist.org/t/clean-up-currently-available-lexicons/12453
https://github.com/inaturalist/inaturalist/issues/2675
https://forum.inaturalist.org/t/require-approval-for-addition-of-new-lexicon-for-common-names/9830