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How to customize my form so that the validation happens on both the first slide and the second?

Hi everybody,
I am trying to set up a validation of the form on the website i’m working on but so far the only validation i could set up only happens on the second slide, which is the submit phase of the form. 2021-02-12_09-42-41 here is a link where i show in specific what I am trying to achieve. In short, i’d like that pop up telling me to fill all the remaining folders on the second slide to work the same on the first slide as well, and therefore the functionality of hitting the ‘next’ button, i would like it to be dependent on the state of the folders, which I’d like to be fill before moving to the next stage.

Please help !! thanks in advance