What does the error code -50 mean?

What does error code -50 mean? I got this while performing a straight copy from my Western Digital 2TB NAS (connected via a 10' Cat6 Ethernet cable). I am copying folders one-at-a time because of the total size of the copy.

What am I missing? I had already copied several large sized folders with no problem. Why am I doing this, you ask? Simple: the WD NAS drive is not formatted in APFS format and, as such, will not allow me to delete items because "they are still in use". The one [particular fie was created in 2011 and has probably not been accessed since it was created (2011). I'm going to try and "piece meal" the copy to find out exactly what file is causing this error code -50. Until then, suggestions?