that's because in old roblox games the place files are stored in "XML" format which is human readable text. new roblox games are in a custom binary format which old roblox clients cannot read. The way to fix that is to open the rbxl file in modern roblox studio, then save it as an .rbxlx file.thanks
Also, once its saved as .rbxlx rename the extension back to rbxl or old roblox might fail to recognize it's file name