u have to convert rbxl file to rbxlx which is in xml format to do this go to studio, then click File > Save File As > change the file extension to rbxlx.
Then u use a roblox place converter for it to compatible with old clients i recommend this one: https://github.com/BakonBot/legacy-place-converter
once its done u can check whether it works by running the file in old studio clients. most of the time it should work tho. then upload the game and youre good to go