I can't open C++ project on Unreal Engine 4.14

[Cultrarius]

It says what the problem is right there in the log:

@merton11;652615

UIDemo2GameModeBase.cpp.obj : error LNK2001: unresolved external symbol "public: virtual bool __cdecl AGameModeBase::SetPause(class APlayerController *,class TBaseDelegate<bool>)" (?SetPause@AGameModeBase@@UEAA_NPEAVAPlayerController@@anonymous_user_e71e0d8a?$TBaseDelegate@_N$$$V@@@Z)
UIDemo2.generated.cpp.obj : error LNK2001: unresolved external symbol "public: virtual bool __cdecl AGameModeBase::SetPause(class APlayerController *,class TBaseDelegate<bool>)" (?SetPause@AGameModeBase@@UEAA_NPEAVAPlayerController@@anonymous_user_e71e0d8a?$TBaseDelegate@_N$$$V@@@Z)

Basically, you reference AGameModeBase::SetPause, but the linker cannot find the implementation for it.
There are two possible cases:

1. You called it somewhere: remove or substitute that call to see if that solves the problem
2. You created a child class from AGameModeBase which is missing the SetPause function. Go and implement it in your class.