I am having the same problem.
Seasonal Challenge "Antiquarian VII" is incomplete (Relic is not placed, Vex Killed = 100%)
Sails of the Shipstealer is on Step 56 of 62. (Wait until next week)
When I approach the table holding the relics, I am not prompted to place the 7th relic.
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I saw a few Reddit posts suggesting that this can happen to accounts when you haven't deposited a relic that you are holding and you start the seasonal quest line with another character. A few of them reported that when they caught up, the relic was eventually able to be placed.
I am almost at step 30 on my second character, but this is taking a long time for a "maybe" solution that seems like it might just break things for a second character.
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It took a while, but my second character was able to get the relic by advancing the story.