I could not Activate Elgg Landing Theme

Hi! I cannot activate the Elgg Landing Theme. It shows

Plugin [custom_index] with version: 4.3.5 conflicts with constraints *

 

I'm just new here and I don't have a strong background on web development or software development. Well, I don't have any at all and this is for a school project. I would appreciate any help on what I should do. Thank you!