message does not appear

There’s a VERY outdated jquery version.

and how do I upgrade? What change?

all other plugins are updated

It’s your theme, not the plugins.

but if I change any chance that the theme does not work any more. how do I?

I solved

I still need a hint. in this site I upgraded jquery, but do not see the message. Why?

https://www.hotelmardelplata.it

Thanks in advance!

Same problem. The jQuery shipping with WP is deregistered and custom, newer version used. However jQuery 2.x is not fully compatible with 1.x

In other words: always use jQuery insluded in WordPress.