I am the creator of the RDR2 Texture Toolkit.
You are probably unable to convert some textures to ytd files because the tool is unable to accept some file texture formats. The tool accepts dds texture files, but there are different versions of dds texture files. This tool is basically the GTA5 Texture Toolkit, but with a ytd file converter that converts GTA5 ytd files to RDR2 ytd files. RDR2 has some dds texture file versions that don't exist in GTA5. You are probably trying to convert one of the dds files that GTA5 does not have, causing the tool to not recognize the file.