If you check your journal, you will find the Tales of Tsushima you have completed. This is likely your last trophy in the game. You should complete all the story missions, clear the red camps to uncover all of the map fog, then complete any outstanding Tales (silver) that you have on the map. You should then have 2 remaining Tales that do not appear on the map:
The first is "The Cost of Fear" and is started in the center of Kamiagata, northeast of High Point Guard Post. You will see a black building on the map. You'll know you have found the right building if you hear a person coughing inside; go to the door to trigger the quest.
The second is "A Debt Repaid" and is started in eastern Toyotama, near Yarikawa Stronghold. Head north from the Stronghold and you will see thick forest on the east side of the river; explore in there and you will find a lone bandit near some dead bodies on the path. He starts the quest.
Credit to this page for handy links for all the Tales including screenshots of the starting points: https://www.powerpyx.com/ghost-of-tsushima-all-tales-of-tsus...