Are you in
PANAMA ?
Updated reasoning thanks to op - my original reasoning is below, but here is the correct interpretation:
"You'll have to 'know why' clues that the definitions for the prefix, infix, and suffix all clue words which rhyme, but need the last 'y' dropped to form the final word:
My prefix is on a roll: PAY - Y = PA
My infix is not on board: NAY - Y = NA
My suffix isn't so sure: MAY - Y = MA
Together we get 'PANAMA'!
Original reasoning:
My prefix is on a roll.
"On a roll" can mean "employed, being paid, collecting salary." "PA, or "per annum" is a way of indicating one is being paid and collecting salary on an annual basis. So the prefix is "PA".
My infix is not on board.
"NA" could be an abbreviation for "Not applicable/not available", meaning I am not on board with something where normally one might request a "Yes" to indicate being on board. It could also simply be a homophone for "Nah", also a negative response to a request to be on board with something.
My suffix isn't so sure.
The suffix is "MA". I am also not sure about this; my best guess at this time is that it is a homophone for "Meh", which is an excellent response to something you are not so sure about. E.g. "Would you like to go see that movie?" "Meh; I guess, if I have to. But I'm also fine skipping it."
My prefix, infix, and suffix all rhyme, but you'll have to know why.
PA, NA, MA rhyme if you pronounce them the same. Not sure why I'd have to know that.
Hints:
You can find me on a globe.
PANAMA is on a globe.
I suppose this hint might unlock something central to where I am.
PANAMA is an isthmus in Central America, and this hint is an acrostic for "I S T H M U S ctwia".
Where am I?
This puts you in PANAMA!