Yes you can Ready an action you currently cannot do
The rules for Readying say:
First, you decide what perceivable circumstance will trigger your reaction. Then, you choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it.
Readying doesn't involve doing anything at all at the time that you Ready. All you have to do is choose "the action you will take". Decide a trigger and decide a future action, there are no other requirements other than that.1
Or, as @RyanThompson says in a comment below:
The key idea is that the ready action means you're doing nothing now and maybe doing something later, and you can always do nothing right now.
Thus, there is no rules or logical reason why you cannot ready an action "to attack with a sword when a receive it from my ally" or "breath fire once my ally casts dragon's breath on me".
Of course if the thing or ability your action depends on fails to get to you before your next turn you have wasted your action, so this is even riskier than a normal Readied action.
Exception: You cannot Ready a spell you cannot cast
Readying a spell requires you to start casting that spell as part of the Ready action.
When you ready a spell, you cast it as normal but hold its energy...
If you do not meet the requirement to start casting spells (or that spell in particular) then you cannot Ready a spell. It is good to note that you don't have to choose a target or even have one in range to start casting the spell.
1 - You can describe the Ready mechanic narratively as taking some preparatory action (holding a sword above your head ready to strike, getting into a sprinter position to get ready to move, etc.) in your game, but that is not dictated or even implied by the rules. The rules simply state the mechanics of taking the action and let the DM and players find a narrative that work with it. Following the rules and creating a compelling logical narrative can sometimes be at odds with each other and nothing in the game says that they have to get along all the time.
It is up to the DM/table to decide where to draw that line and where and how much to tweak rules they see as crossing it. Regardless, the rules say what they say.