The reduce function has stopped working.
Whenever I attempt to reduce 4 or fewer staves I receive this message: Composing Tools/Reduce::Run:95:Parse error near "mainstaff"
This kind of error (parse error) says that something is wrong with the plugin file. TI checked the shipping version of Reduce as of Ultimate 2021.2, and it has no parse errors in Run.
In suggest to me that you either have an earlier version of Sibelius with a bad version of the plugin, or as Robin suggests, you have a private copy of the plugin.
If you do not intend to have a private copy, I suggest to go to File>Plug-ins> Edit Plugins, and type Reduce into the Find box and click on Find.
You should see Reduce in the category "Composing Tools". If you see "(user copy)" next to the name, and did not intentionally made a private copy of Reduce, you can select the entry and click on Delete, and it will remove the private copy and restore the shipping version, which will no longer says (user copy).
In my screen shot Reduce does not have (user copy), though a lot of other plugins do, because I mess with plugins a lot.