Please check first the content of your _PQXX_INCLUDE variable (e.g. inside the cache file) to see if the find was correct.
I assume that you need to specify the correct file name for the find, which is: pqxx.h and not just pqxx
Please delete your build directory (or at least the cache file).
The variable needs to be a directory, not a file, and this is not the result of this find_path command. It probably was already set by earlier experiments or you have another find with the same variable.
This will find the path, which you need to add to your include directory. Which is in fact D:/libpqxx/include
I assume this is just an example to understand the find command. If you know the path already and hardcode it, then you don’t need to search for it in the first place.