As you might be aware, power levels were rescaled as of Shadowkeep with 750 being the new '0'. Some old characters, expecially ones that did not complete the original campaign, might be stuck with either gear that shows as 0 power, or are completely missing a heavy weapon, meaning their overall power level is too low to damage their 750 power opponents. I can see your Titan has power less than 300 but your Warlock looks ok. On those characters, go to your Collections and pull them some 750 gear to fill any slot that is currently empty, or containing a bugged piece of gear. That should allow you to complete the prologue.