Cannot figure out how to progress.

[Jeeqbit]

Enkidoh explained it, but by that quest, you should at minimum have equipped your Item Level 90 Warrior job gear.

While it's important to do the MSQ (fireball icon quest), you also need to do class/job quests, which the game reminds you of every 5 levels in the chatbox.

Warrior quests (which Marauder upgrades to) take place just north of Costa Del Sol in Eastern La Noscea. It's a blue quest.

Upon completing the level 50 Warrior quest, you should have a coffer in your inventory. If you select Use on it to redeem it, you should have some Item Level 90 gear, which you can equip to resolve the issue.

To equip it quickly, you can open the character window and click the Recommended Gear button:


Now, you can actually get better gear by clicking the blue bag NPC at Gridania, in exchange for Allagan Tomestones of Poetics (you get these from level 50 dungeons). I recommended starting with accessories (ring, bracelet, necklace, earrings) because these will need upgrading the most: