Because tlskinandcapemodforge121jar is an ancient file (over a decade old), you must exercise .
Press Windows + R , type %appdata%\.minecraft , and find (or create) the mods folder. file name tlskinandcapemodforge121jar
Files with opaque names are common in modding communities. Best practices before running such a JAR: try disabling it first.
May be redundant if your launcher already includes "TL" versions of Minecraft in the version list. Security Note TL Skin and Cape для Minecraft 1.21.4 file name tlskinandcapemodforge121jar
The mod is incompatible with another skin mod (e.g., CustomSkinLoader, OfflineSkins) or with OptiFine for 1.21. Fix: Remove all other skin/cape mods from the mods folder. If you use OptiFine, try disabling it first.