Deathless Ancient's last ability can be activated only while it's in your graveyard.

To activate the last ability, you may tap any untapped Vampires you control, including ones you haven't controlled continuously since the beginning of your most recent turn. (Note that tapping the creature doesn't use [the tap symbol].)

Once you announce that you're activating Deathless Ancient's last ability, no player may take other actions until the ability's been paid for. Notably, players can't try to stop the ability by tapping or removing Vampires you control.