You are not using a supported Time Machine configuration. To learn how to use Time Machine read Use Time Machine to back up or restore your Mac.
Despite their manufacturers' insistence to the contrary, there are no non-Apple NAS devices that will reliably work with Time Machine. If you want to use your Buffalo NAS device to back up your Mac's contents, then use a backup strategy other than Time Machine.
The exhaustive list of devices supported by Time Machine amounts to the following:
- AirPort Time Capsule's built-in hard disk (any model)
- External USB hard disk drive connected to a Time Capsule (any model)
- External USB hard disk drive connected to an AirPort Extreme (802.11ac model only)
- A hard disk drive directly connected either to the source Mac, or a Mac running OS X Server on your local network.
That is all.
Use whatever backup device you want, but you should be aware that this site is full of reports of misery from hapless individuals who had been using third party NAS devices for Time Machine backups, only to find that they were incomplete, corrupted, or useless in the dire circumstances in which they were required. Apple won't care if you lose your data while using a Time Machine configuration specifically excluded from their technical support documents.