Are these sentences ok and natural?
Sorry, I can't be of help to you on this.
Sorry, I can't be of help to you about it.

Question

Answer

They're okay, but I would probably use:
Sorry that I can't be of any help to you
or just
I'm sorry that I can't help you

Answer

Sorry, I can't be of any help to you, on this.

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I would simply say, 'Sorry I can't be of more help' or 'Sorry that I can't help you.' But both of yours are fine.

Answer

They both sound natural enough. You can also say:

"I'm sorry I can't be of more help to you in this regard."

or simply

"I'm sorry I can't help you."