Do you have a Facebook account?

Is the question about email sent to you by Facebook?
If yes then the notification email should have a comment link you simply click to open.

People using Facebook can make comments exclusive to only be seen by a select group of friends though if you are in that group you need to be logged into Facebook to access those comments.

Is the question why you cannot make a comment on a Facebook page?
If that's the case then either you are not logged in or do not have a Facebook account.
The two sample images below show the differences. The first sample image shows a comment box because the user is logged into Facebook.
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* Please note again Joe, your email inbox is full yet again and emails are being rejected from your email account. We keep receiving messages saying notifications cannot be sent to you because your inbox is full. I am guessing you are using a very old email service because most email services these days allow a very large total of emails to be saved. Obviously not your email account.

If all your Facebook notifications are being sent to your email then it's no surprise your email inbox is constantly becoming full; flooded with Facebook notifications. There are far better and more practical methods of receiving Facebook notifications.