When the Destiny 2 Invitations of the Nine were first released, players were able to complete them by killing enemies on any planet, not just the specified planet. Bungie quietly fixed this bug after a few weeks, which would normally be fine, but the bug was fixed just in time for an Invitation which cannot be completed because the assigned enemies aren't available on the specified planet. Happily, this Invitation will be fixed on Tuesday, May 7 with the release of update 2.2.2.
As Bungie explained last week, Invitation of the Nine #7: The Tower requires you to kill Hive, Scorn, and Vex in the Tangled Shore. The trouble is, there aren't any Vex in the Tangled Shore. At all. They don't exist. As a result, players couldn't complete this Invitation in time for Xur's next visit, meaning they fell an Invitation behind on the nine-week schedule. The Invitations of the Nine have slowly rolled out an interesting side story about the Drifter and the Nine - with collectible lore to match - so players are understandably eager to reach the end.
The good news is that this ironic setback will only delay the Invitation conclusion by one week for avid players. Even though it wasn't completable, Invitation of the Nine #7: The Tower was still purchasable, meaning most players already have it in their inventories - many of them with all but the bugged step completed. As long as you complete your latest Invitation by the following Thursday - the day before Xur visits - you'll be eligible for the next one. In other words, because update 2.2.2 will arrive on Tuesday, players will have two days and change to complete The Tower and unlock the eighth Invitation. So if you haven't picked up The Tower yet, you can get a headstart on it this weekend and finish it next week.
Speaking of the shady Exotic merchant, here's the latest Destiny 2 Xur inventory.