Presuming you installed TS through Steam, either simply by buying it there or as a key from a third party seller which you activated in Steam, Steam will keep the TS core program updated to the latest version so compatibility with the latest DLC should never be an issue. This isn't entirely magnanimous on Dovetail's part- if everyone has the latest version of the program it encourages users to buy new DLC and makes support easier (ahem). Presuming from your question that you got started in TS with 2021 have you not been updated since? You should have noticed the loading splash, branding and menu background change to identify itself as TS Classic.
