Pure virtual not implemented error

How would I go about pinpointing an issue like this? I’m not even sure if I changed anything to cause it (probably did somewhere but can’t remember)

2>    Running UnrealHeaderTool "" "" -LogCmds="loginit warning, logexit warning, logdatabase error" -Unattended -WarningsAsErrors -installed
2>LogWindows : error : === Critical error: ===
2>  LowLevelFatalError [File:] [Line: 129]
2>  Pure virtual not implemented (UProperty::GetCPPTypeForwardDeclaration)
2>Error : Failed to generate code for SwatterEditor - error code: CrashOrAssert (3)
2>  UnrealHeaderTool failed for target 'SwatterEditor' (platform: Win64, module info: ).
2>): error MSB3073: The command "" "" -waitmutex" exited with code 3.