How to change keyboard shortcuts in Keynote?

Option-i is a "dead key" in the US layout.

It is used as a combination with a subsequent key to type accented characters - for example, â ê î

To get these characters, you press Option-i followed by the respective vowel.

Similarly, Option-e for áéíóú; or Option-n for ãõñ

These dead keys won't work as keyboard shortcuts without another modifier, as in your case.

There may be other such keys. They are visible if you show the keyboard viewer and hold down the modifiers.

For example, here is the keyboard viewer when I am holding down the Option key on my keyboard - note the orange marked keys:

Here they are Option-e Option-u Option-i Option-` and Option-n

There mau be others, depending also on the keyboard layout.

And there may be other keys reserved for other system functions.