I think space should be the problem. There should not be space between the characters or numbers. There is a space between 46 and 51. It should be 46-51 instead of 46 - 51.
Yes, Anjum is exactly right. The space is the problem. Your DOI suffix can be any alphanumeric string. Limited punctuation is allowed, the approved character set for DOI suffixes is: “a-z”, “A-Z”, “0-9” and “-._;()/”.