The mkdir command comes first, else /mnt/userdata doesn’t exist (I suspect this is the case now). Please try in ADB shell …
mkdir -p /mnt/userdata
mount /dev/block/platform/11120000.ufs/by-name/USERDATA /mnt/userdata
If both commands succeed without complaining, you can continue with what was suggested further …
cd /mnt/userdata
find . -type f -size +1000M