You are trying to install a modified version over an official version, or vice versa. You must uninstall the existing version first.
Apps installed via APK must be manually updated, meaning you’ll miss critical security patches. How to Safely Install TikTok Lite tiktok lite version v2151 apk download upd mirror
Help you find a of the TikTok Lite APK if v21.5.1 does not meet your needs. You are trying to install a modified version