Then use whatever Forge version is compatible with the most recent version of Optifine, it's their restriction not ours
Alternately you could uninstall Optifine and be free of that restriction.
The installer log should be in the same place as the installer file, either with the same name as the installer with .log on the end or just called installer.log
In that case I guess try deleting /Users/[surname]/Library/Application Support/minecraft/libraries/net/minecraftforge/forge/1.16.4-35.1.4/ and running the installer again. If that doesn't work provide the installer log.
The only thing you should click on adfocus is the skip button in the top right, but if that prevents you from getting Forge entirely there are direct links in the "All Versions" section, just click on the circle-i next to the file you want.