what we should write

a HR or an HR

please elaborate.. from pronunciation, it seems an HR is correct.
An HR staff member. The word 'H' (aitch) begins with a vowel sound.
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An hr is correct because whenever words begin with vowel sounds, an is used instead of an a even if they are not spelled with vowels.

as h is pronounced like -aytch

another example of this can be an MBA

as M is pronounced like em

In Canada, we say an HR person, not an HR.