error handling in existing system

I want to add error messages to an existing system, the problem is that there is no designated space for them, here are some examples of screens we are facing: enter image description here enter image description here enter image description here

How do you suggest to handle errors?

What I plan to do:

  • Paint the frame in red
  • Add a small X on the top right of the frame (for accessibility)

What I can't decide on:

  • Should I show errors on popover (on click) or tooltip (on mouse over)?
  • Will there be an accessibility problem for screen readers to handle popover or tooltips?
  • Any other ideas?
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