On this page of the Assassin's Creed Valhalla Siege of Paris guide you will find a detailed description of all the steps you need to take to obtain the Joyeuse weapon.
Roman Excavations in the Melunois region can be found at the Diodurum Ruins.
Roman Excavations in Evresin can be found in the northern part of the region.
Roman Excavations in the Amienois are in the Champlieu Ruins.
You will find this chest in the south part of Paris.
This treasure can only be accessed after all the Roman excavations have been collected.