Menu Sub-item sela theme

I recently switched to sela theme, I downloaded it from wordpress.com. The theme overall is working great. One big issue, I am not able to put any sub-items in the menu.
I used to be able to do that easily in my 2015 theme, and the subitems carried to the new theme. However, when I try to add new ones such as “Exercise during pregnancy” and “The big fear: excess weight during pregnancy” to go under “Pregnancy” it simply does not add them.
I can put assign them under pregnancy, however once I click the save menu button they go back to being as tabs on their own.
Please help me fix this issue, I posted a question on www.ads-software.com forums however, no answer.
My website is https://www.onethousanddaysoflife.com/