Can't join my girlfriend's modified world

my girlfriend is playnig on her minecraft world with some fantasy like world, with fairy, cottage core type of mods and she asked me for a help on terraforming an area on her world but when I pressed join, the mods on her world that I have to download is downloading then suddenly in the middle of the download it stopped for a bit then it says I can't joining the world and have to check my internet or the host, but our internet connection is ok. we try everything we can to play together at her world but w always get the same message that we can't play together or I can't join her world.