It might be because you need to specify the BLASLAPACKINCLUDEFLAGS as well. There should be a flag starting with -I followed by the directory where your header (.h) files are for your LAPACK installation.
Please check the folder to make sure it has header files such as lapacke.h (or some set of relevant headers).
Another thing to check is whether you are setting PLATFORM=lapack correctly. Like, for example, is the PLATFORM variable set again to a different value further down in your options.mk file? The reason I ask is that if PLATFORM=lapack is set correctly then things like LAPACKINT should be set by ITensor itself. So if those aren't getting set then maybe the PLATFORMlapack variable is not being defined.
Finally, please make sure the options.mk file you made is from a recent version of ITensor. If you upgraded from a much older version of ITensor, then you should make a brand new options.mk from scratch from the most recent options.mk.sample file. (If you are just installing ITensor for the first time I assume that's what you did.)