Element not interactable for a button to click

Near the top of your steps:

String js;

(EDIT) Enregistrer:

js = "document.querySelector('#twittcompta-creditnote-sentat-modal input[type=submit]').click();"
WebUI.executeJavaScript(js, null);

(EDIT) Valider:

js = "document.querySelector('#twittcompta-invoice-sentat-modal input[type=submit]').click();"
WebUI.executeJavaScript(js, null);

When you have it working, you should investigate WebUI.enhancedClick() which will fall back to trying JavaScript without you needing to write the JavaScript yourself.