Parts of my landscape keep dissapearing in Play Mode???

Development World Creation
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1

Everytime I go into “Play Mode”, huge chuks of my landscape keep dissapearing. Does anyone know why this is. I am completely new the Unreal Engine, and I am not sure what is causing this probem. Does anyone know a solution to this?

unreal lanscape problem
unreal lanscape problem2634×1486 2.14 MB

^Parts of the landscape that are missing in “play mode”

normal viewport mode
normal viewport mode1920×1153 191 KB

^How the landscape should look (this is the normal viewport mode)

2

i have same problem too. And i have same message too “PHYSCIAL MATERIAL NEEDS TO BE BUILD”
It can be source of this problem. I didnt try to build yet but you can