Oke, validate_save_post seems to work.
function my_acf_validate_save_post() {
acf_add_validation_error( 'acf[field_64ad703e1fab1]', 'Please check?? this input to proceed' );
}
add_action('acf/validate_save_post', 'my_acf_validate_save_post');
Keeps me wondering though why add_filter (‘acf/validate_value’) is not working…
