The “uni” in unistd stands for “UNIX” – you won’t find it on a Windows system.
Most widely used, portable libraries should offer alternative builds or detect the platform and only try to use headers/functions that will be provided, so it’s worth checking documentation to see if you’ve missed some build step – e.g. perhaps running “make” instead of loading a “.sln” Visual C++ solution file.
If you need to fix it yourself, remove the include and see which functions are actually needed, then try to find a Windows equivalent.