Why can't I open a tab with a html-button while being on the chrome web store?

I have somewhat of a solution: for your Google webstore link change popup to something else like popupWindow, so you'll have:

<button id="button1" onclick="window.open('https://www.facebook.com/','popup','width=700,height=300');"><strong>CONTINUAR</strong></button><br>
<button id="button2" onclick="window.open('https://www.google.com/','popup','width=700,height=300');"><strong>CONTINUAR</strong></button>

<button id="button3" onclick="window.open('https://chrome.google.com/webstore/detail/dark-mode/dmghijelimhndkbmpgbldicpogfkceaj?hl=de','popupWindow','width=700,height=300');"><strong>CONTINUAR</strong></button><br>
<button id="button4" onclick="window.open('https://www.9gag.com/','popup','width=700,height=300');"><strong>CONTINUAR</strong></button>

You will have actually have two different popups with this code.

About the explanation, I am a bit in the dark with this one. I think Chrome is preventing executing JS on https://chrome.google.com/webstore/* as a security measure. You can read more about similar issues here and here here.

Addition (after having some thoughts about it):

This is probably a smart move by the developers of Chrome. By not allowing any JS to alter any of the pages on https://chrome.google.com/webstore/, they are sure that also no extension can possibly alter this page. Immagine if you install an extension that does the thing in advertises quite well so it will get a positive score, while it also alters the page of the extension webstore. It could trick users into installing additional (hacky/adware-ish) extensions or software that would infect the users browser or computer.