❔ Not sure what this error means, i'm new to c#

Severity    Code    Description    Project    File    Line    Suppression State
Error    LNK2019    unresolved external symbol "class FrameX::Application * __cdecl FrameX::CreateApplication(void)" (?CreateApplication@FrameX@@YAPEAVApplication@1@XZ) referenced in function main    FrameX    C:\Dev\FrameX\FrameX\FrameX.obj    1