To answer #1, no "metadata file" is created unless the original is a RAW photo. Otherwise, the metadata is actually written into the photo file header.
To answer #2, I don't know the answer but I do know it has been discussed many times in this forum and if you search for the answer, I'm sure you'll find it. Just guessing, perhaps you have a permissions problem where the folder that the photo is in does not allow you WRITE permission.
#1. yes, it is a RAW file, but still not writing to metadata to disk.
#2. i checked few threads, and it was referring to Windows OS or older version of Mac - not related to Sierra.. is there any "logs" of lightroom to check is there any folder permission issue (for info - it use to work few weeks back, but not now)
1. Do you have write permission in the directory/folder where the photo is stored?
2. So you didn't try the solutions given there? Please try them!