Why is my self built LiFePO4 battery pack not charging?
There is no detailed manual for the BMS board, so it could work as it is supposed to, but we just don't know how it is supposed to work.

So this question may not be answerable - we can't know but we can guess.

It clearly has latched into a state that the 18V 2A supply is disconnected from the battery. But what we don't know how it ends up in that situation, whether the BMS is already cutting the battery off, or does it only trigger the cut-off when the 12V 2A supply is connected, and continues cut-off until it is removed.

One guess is that when the 18V 2A power supply was connected to the empty battery pack with about 14V, the pack will load down the power supply output to 14V and will draw more than the 2A it can supply, so the power supply output undervoltage/overcurrent protection triggers in and shuts down. That may have also caused current to flow from batteries to the power supply, so the BMS may have triggered some protection to cut off the batteries from the supply, and it will be cut off until charger voltage is removed.

Another guess is that the BMS considers the batteries drained and has cut off the batteries permanently, so no current can go in or out from the battery now, until they are trickle charged to exceed the cut-off voltage.

You would really have to present more data to know what is happening, for example if you had an oscilloscope, you could probe what happens when you connect the two together.

Except that you should not be connecting them together in the first place, and the BMS has maybe performed it's job to prevent catastrophic failure with flames and explosions.