It looks like you've dug down-stairs on the level below. Down-stairs need an up-stair below them to work - or an up/down-stair if you want to go down additional levels.

If you don't want to dig around, you can solve your problem in this case by building an up/down-stair on one of the tiles on the lower level (through the Constructions menu). Make sure you use material that your dwarves can get to to the construction job, though (so not one of the nearby gabbro stones).

It looks like you've dug down-stairs on the level below. Down-stairs need an up-stair below them to work - or an up/down-stair if you want to go down additional levels.

If you don't want to dig around, you can solve your problem in this case by building an up/down-stair on one of the tiles on the lower level (through the Constructions menu). Make sure you use material that your dwarves can get to for the construction job, though (so not one of the nearby gabbro stones).