Please help! What is the name of the condition when you are missing an L5 bone in your spine?

Not sure exactly. What you are asking. There is no condition where you are missing l5, but there is a condition where your L5 can be sacralized. In other words, your L5 is fused with your sacrum. This can appear you do not have an l5, but really it just appears to be part of the sacrum. There is nothing pathologic about this, as many people do not know they have it until by chance they have an xray. Hope that helps.