Hey there, @mouna1985
Bo here from Shopify Support!
Thank you for getting in touch regarding this. If you go to Online Store > Themes > Current Theme > Actions > Edit Languages you should see a notice here that the newsletter confirmation translation is missing. You can fill this with a generic thank you message. If this does not solve the issue, let me know. If this is a Shopify supported theme, I will be able to reach out via email to authenticate you as the owner and take a closer look. If it is a third party theme I would recommend reaching out to the theme developer. You can do so as per this guide.
As this is regarding your newsletter I just wanted to share some guides on email marketing.
All the Best,