Condensation under bricks in doorstep

[Nelsons]

I'm putting in an internal doorstep inside our kitchen door. Laid the bricks out a week ago to check it was the right size. I lifted them this morning preparing to mortar them in, and there is condensation underneath. I've never laid bricks before; do I need to do anything to prevent the condensation or will it right itself once the bricks are properly laid? I'm planning to tile over them with slate.