How do I cancel the shortcut keys Ctrl+F2 and Shift+F2 when text is selected?

Shift + F2 is currently assigned to “Find Next” in InDesign, and Ctrl + F2 doesn’t have a default function in most versions.
If you want to use those shortcuts for something else, you’ll need to remove or reassign the existing “Find Next” shortcut in Edit → Keyboard Shortcuts. Otherwise, InDesign will keep triggering the original command instead of your new one.

When you press those keys in order to put them into a shortcut; if already used, it says so.

InDesign > Edit menu > Keyboard Shortcuts

First make a new custom set of your Keyboard Shortcuts.

Next, find the command you want to make a shortcut for.

Or re-assign an existing used shortcut to command you want to trigger.

Notice that when editing a shortcut, there is a small field labeled as the Context field, where you can limit a shortcut to only work with text, for example.