Adverbial phrase
 

I found the following example in one of my English books:
Things are a little up in the air at the moment. If you pencil the meeting in for Wednesday, I’ll get back to you before packing up tonight on whether I can make it or not.
Could anyone tell me whether on whether I can make it or not serves or modifies get back to you or packing up tonight?
Thanks very much.
PS: Please notice me for any of my mistakes (grammar, word choice, etc.)
Now I've realized the answer. The sentence means the person will reply about if he can attend the meeting on Wednesday before packing up.

Re: Adverbial phrase
 

I think I was just an idiot that I didn't see the answer. on whether I can make it or not obviously modifies get back to you; the person will answer if he can attend the meeting on Wednesday.
Thank you all for your attention.

Re: Adverbial phrase
 

Edited : I've just seen your addition in red. You must have posted it just before I wrote this. Anyway, I'll leave it as it might be useful to someone else.
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No - whether I can make it or not means "whether I can attend the meeting or not".

"make it" is a fixed phrase, with the meaning to fix an appointment or "to be available for an appointment" or ". So for instance :

A : Do you fancy going to the cinema tomorrow?
B : Well, I can't make it tomorrow, but what about Friday?

ie I'm not available to go to the cinema tomorrow, but what about Friday?

Similarly...

A : I need to talk to you about the ABC project.
B : OK - things are a bit busy at the moment, though. Let's make it Wednesday.

ie : Let's fix the appointment for Wednesday