Here string in while loop throwing syntax error

Question

I really don't understand too much about this whole here-string thing, but according to all my research this should work. And it did. A while ago. The intended operation is to download randomly generated SALTs from the wordpress api, and insert them into the wp-config.php file (opened in $wpconfig, referenced by sed). They are already predefined in the file, but empty.

It throws a syntax error upon execution. It complains about the marked line:

echo 'Replace SALTs in wp-config? [yes/no]'
read saltsOption
if [ "$saltsOption" = 'yes' ]; then
        echo 'Starting...(this might take a while)'
        SALTS=$(curl -s https://api.wordpress.org/secret-key/1.1/salt/);
        while read -r SALT; do
                SEARCH=$(echo "$SALT" | cut -d \' -f 2)
                REPLACE=$(echo "$SALT" | cut -d "'" -f 4)
                echo "$SEARCH -> $REPLACE"
                sed -i -e "s/\(.*'$SEARCH', *'\)\(.*\)'/\1$(echo $REPLACE | sed -e 's/\\/\\\\/g; s/\//\\\//g; s/&/\\\&/g')'/" $wpconf;

        done <<< '$SALTS'; # it complains about this line! *Syntax error: redirection unexpected*

        echo 'Done.'
        echo '';
fi

Am I not understanding something here? it's running on Ubuntu 18.04.4

Thanks guys!