I regret being unable to help you today.

[diamondcutter]

1. I’m sorry for being unable to help you today.
2. I regret being unable to help you today.

These two sentences were written by me. I think they are exchangeable. Am I right?

 

[Tdol]

I'm sorry I can't/am unable help you today sounds more natural to me.

 

[5jj]

1. I’m sorry for being unable to help you today.
2. I regret being unable to help you today.

They are both possible if you were unable to help this person earlier in the day.