can a/an modify unccountable noun?

My father was not a tall man but he was able to command a room. He had a presence about him, the solemnity of an oracle. His hands were thick and leathery—the hands of a man who’d been hard at work all his life—and they grasped the Bible firmly.

I searched with an answer that a/an X about is a fixed phrase, but I still don't understand as presence is unccountable, why a/an is used here?

You have had solid answers to your question. Let me take a slightly different tack.

First, the word presence here has a special sense,

Also ‘a’ is also being used in a special way. It is not like ‘a pear’ or ‘a sports car’. it is in a deeper way ‘indefinite’. It has the sense of ‘some undefined’ or ‘some unstated’... Ancient Greek and Latin had words that did this job. In Greek the word was ‘τις’ (tis). In Latin it was ‘quidam’. Both were enclytic (followed the noun they qualified.).

Praesentia quaedam or παρουσία / επιφάνεια τις

School kids of my generation were taught to translate these as ‘a certain, conveying something undefined or (literally)* indefinite*.

So because the presence is undefined, it cannot be counted. Perhaps the presence of one or of several *whatever-it-ises *. All the character is aware of is a presence.

This noun like many others can be both countable and uncountable.

In your case it is countable.

According to Oxford English Dictionary (https://en.oxforddictionaries.com/definition/presence):

Presence

1.1count noun 

A person or thing that exists or is present in a place but is not seen.

‘the monks became aware of a strange presence’

As about collocation with 'about', look at the example from Reverso.context.net:

"If by that you mean Mr Cochrane is manly, yes, he has a certain presence about him."