Thank you so kind for paying attention to my question. I followed your instructions. Although the 'Page Attributes' box was already ticked, the 'Parent' element suddenly returned. So the problem seems to be solved. Thanks again.
There's another thing: I have added a 'Home' page (marcoderks.wordpress.com/home) but there is already another 'Home' page (marcoderks.wordpress.com) mentioned in the menu. However, I can't find the latter so when I go to marcoderks.wordpress.com I see the text:
Apologies, but no results were found for the requested archive. Perhaps searching will help find a related post.
But I can't figure out how to solve this. Do you know that?