You need to put add_filter() outside of the function
function my_acf_prepare_field( $field ) {
// Lock-in the value "https://instagram.com/".
if( $field['value'] === 'https://instagram.com/' ) {
$field['readonly'] = true;
};
return $field;
}
add_filter('acf/prepare_field/name=instagram', 'my_acf_prepare_field');