It is a bit odd, but you do not start out with the ability to build floors in Grounded. This is an upgrade you will need to obtain if you want to not walk in the dirt around your pad all the time. This will also enable you to make more floors in your base, so it's something you are going to want to get pretty quickly if you are a builder.
To make floors in Grounded, you need to finish the two story quests that you can do in the early access. One is fixing the mysterious machine, and second is exploring the explosion near the big oak tree. Once you've get them done, you can purchase the ability to craft floors from BURG.L!
If you can't see the tree, then try to get up somewhere high and look for it. It's on the northern most point of the map before you get to the river area. It's directly opposite of the house, so check your map and you should be able to get to it.
Once you are at the tree, you will want to be careful of the Wolf Spiders that spawn in the area. Head around to the northern portion of the oak, and look for the entrance to the science lab which looks like this:
Head inside of here and you will find a projector displaying some crazy geometric shapes, a tape that you need to collect, and behind that there's a light puzzle you will need to solve.
Go over to the six button and just keep tapping on them until they all light up green. You might be able to figure out how it actually works, but I was able to just brute force it. Once they are all green, hit the lever on the left and it will unlock the door. Inside of here you will find a robot who needs a bit of fixing, but you just need to interact with him. Be careful in here because there's an enemy robot that will attack you that you need to dispatch.
Once you are through all the dialogue with the robot you will then be able to get to the part where you can upgrade some of your building options. However, you will need to collect a certain amount of Raw Science to unlock it. This is obtained by researching the various new items you find at the Field Stations. You can also get this by completing quests for BURG.L. You might have quite a bit already, but then again you may not. Grab all the quests from him, they usually just require you to slay bugs.
Once you've got the 1,000 Raw Science, you can then go back to BURG.L and purchase the Multi-Story Bases upgrade. You can now create Stairs, Floors, and Triangle Floors. Stairs requires you to use Weed Stems, so make sure to check out our guide on how to get them!