In the Thrumbo Wool info tab there is a grammatical error.
What is written : Thrumbo Wool clothes is a extravagances.
How it should be written : Thrumbo Wool clothes are an extravagances.
Thrumbo wool isn't a vanilla thing.
So you should report this to the mod author of the mod that add these wool ! :-)
Yup that ain't vanilla...
It's from a mod my friend! https://steamcommunity.com/sharedfiles/filedetails/?id=859466666 (https://steamcommunity.com/sharedfiles/filedetails/?id=859466666) might want to post a comment on the page there my friend but they aren't the type with the best english. It's best to edit the mod yourself with an text editor... ;)