sh - unexpected operator in if statement

In the following two lines I get this error?

What is wrong?

Debian Buster

my.sh: 101: [: !=: unexpected operator

my.sh: 103: [: !=: unexpected operator

if [ $CONTINUE != "y" ] && [ "$CONTINUE" != "n" ]; then

elif [ $CONTINUE = "n" ]; then

update

echo "\nContinue downloading? [y/n]"
read CONTINUE

#   Error: Invalid argument
if [ $CONTINUE != "y" ] && [ $CONTINUE != "n" ]; then
    error "Invalid argument"
elif [ $CONTINUE = "n" ]; then
    echo "\nDonwload terminated!"
    exit
fi