Got Ancient Doom prophecy, but I do not have Doomfletch.

In the SSF Standard league, Navali gave me an Ancient Doom prophecy.
"You will defeat the Vaal Oversoul while holding Doomfletch."

But I do not have Doomfletch.

What does this propecy mean?

Perhaps "Just go to Act2 pyramid, and you WILL find (as loot) a Doomfletch on the floor (before meeting Oversoul)"?

Or, may it mean "You have a RoyalBow (at least 1). Just use an OrbOfChance on a RoyalBow and you WILL get, luckily, a Doomfletch. Then put it in your inventory, kill VaalOversoul, and you will be rewarded by some XY loot."?
(By the way, what loot?)

Or, it is a dead (unfulfillable) propecy, I have to seal it (sacrificing another coin).

The prophecy will upgrade a non-corrupted Doomfletch if it's equipped or in inventory at the moment you kill Vaal Oversoul. Getting ahold of one is your responsibility, but it doesn't matter when or how you do (it doesn't matter that you got the prophecy before the item).

If you talk to Navali you can seal the prophecy and save it in your stash for later if you'd like, it will cost a few silver coins though.

May it be an option (in SSF!) to use some other user's Doomfletch? Partying, chatting, borrowing?...

(On the https://pathofexile.gamepedia.com/Partying page "SSF" is not mentioned...)

But if you check the SSF page...

Solo Self-Found (SSF) is a feature that marks a character to be unable to interact with other characters, which disables features like trading or partying.

Here's a recap of what others have mentioned:

The prophecy means that *if* you have a Doomfletch and are carrying it in your inventory when defeating the Vaal Oversoul, then it will be upgraded to the Doomfletch's Prism bow.

It *does not* guarantee that Doomfletch will drop for you just because you have the prophecy. You have to obtain the Doomfletch yourself first. As you mentioned, it can be obtained with an Orb of Chance, but also through some divination cards, or just randomly dropping.

If you don't have a Doomfletch and you want to save the prophecy for later after you find one, then you can seal it and come back and activate it later.