how to Using bash to generate random IPs in Curl

You're quoting incorrectly. You can try:

curl --header "X-Forwarded-For: $(printf "%d.%d.%d.%d" "$((RANDOM % 256))" "$((RANDOM % 256))" "$((RANDOM % 256))" "$((RANDOM % 256))")"

You can get rid of the inner quotes and even the printf entirely:

curl --header "X-Forwarded-for: $((RANDOM % 256)).$((RANDOM % 256)).$((RANDOM % 256)).$((RANDOM % 256))"

However, whether the target site accepts the X-Forwarded-For is another matter entirely. Setting this header does not actually hide your own IP from the target site.

I will suggest this design

shuf -i 0-255 -rn4 | paste -sd.

But it loses printf since the first value in IP should not be equal to 0

printf %d.%d.%d.%d $((RANDOM%255+1)) $((RANDOM%256)){,,}

First the shell opens the "Brace Extension" and then substitutes the values of the variables