You need to place certain objects in your map before it will support invasion slayer. I found a guide in a forum post by EternalEnmity. It says:
Here is a halfway decent guide in making Invasion Slayer maps.
You need to have at MOST 6 initial spawn points for each team. They need to be labeled INVASION and set to the proper team.
You need backfield spawns. A backfield Spawn is an area that is available to either all fireteams or some of them in all phases.
(Fireteam-each team is split into 3 mini-teams. Each player in a
mini-team has a partner they can choose to spawn by, those miniteams
are called Fireteams) A good number of backfield spawn zones to put
are 3. Set each spawn zones spawn sequence to 0, 1, 2 or 3. 0, 1 and 2
are each individual fireteam. Setting it 3 will make it available to
all fireteams. You must label it INV_SLAYER_RES_ZONE
You HAVE to have at least 1 Drop zone. This will be labeled as INV_SLAYER_DROP. Set all the spawn sequences to either the same, or
chronological order. This will decide which ones spawn before the
others. Set all the same for random choosing. Now, I'm not exactly
sure whether or not you can choose which weapons came out of a drop,
or in which phase. But I don't see why you shouldn't be able to.
Unlike The normal Invasion, you cannot phase or gate objects. This
means you can't have objects spawn and/or delete themselves upon
Blue team is Elites, Red team is Spartans.
Vehicles labeled with CREATED_VEHICLES make it so a vehicle is
indestructable for 5 seconds upon spawning.
Never EVER use the labels INV_PLATFORM or INV_MANCANNON as those or
specific to the huge gravity lift, and the mancannons, respectively,
that spawn in Spire in the 3rd phase of Invasion