Your website shows a valid SSL certificate indicating that your site is secure except when a login form is used. During logins or form submissions, a webpage is shown that says, "The information you're about to submit is not secure," and asks if users want to proceed or go back.
The webpage users might see during this will look something like the following:
As of Chrome version 87, this full webpage prompt will occur when users try to perform a form submission on a "mixed form" - that is, a form that does not submit over HTTPS.
You can read more about the change in behavior on this Chromium blog post: Protecting Google Chrome users from insecure forms.
To resolve this, the site developer will want to review their code to ensure that form submissions happen securely over HTTPS.