Follow this guide, once completed your steam deck should be fixed..

https://steamcommunity.com/app/1675200/discussions/0/3269061071528807540/%3Fctp%3D6&ved=2ahUKEwi5g7vygN76AhX9QUEAHdNIA8IQFnoECAkQAQ&usg=AOvVaw1hyz_YZ7PWzPLiBPCoaq0t

Here's the process:
1. Turn on your Steam Deck
2. Hit the 'Steam' button and go to 'Power' and select 'Switch to Desktop Mode'
3. In Desktop mode, click the Start button in the bottom left corner(Steam logo)
4. Select 'System', then off to the right select 'KDE Partition Manager'
5. Off to the left under 'Devices' it will show 2 options: 1 will be 'mmcblk0' and the other will be your internal SSD(named after the drive manufacturer. Mine says "KINGSTON")
6. By default the SD card(mmcblk0) should be selected. If not, select it.
7. In the middle area of the program it will show your partitions. It will show the SD card partition and it should have a little blue box next to 'ext4' under type, a lock icon under mount, and the size and used space.
8. Using the right trackpad hover over it, then using the left trackpad click it. A drop-down menu will appear.
9. Select 'Unmount'
10. Click it again with the left trackpad and select 'Delete'. In the bottom of the program in the 'Pending Operations' box make sure it says "Delete partition". At this point you can undo this if it doesn't. Make sure it doesn't say "Shred", it should say 'Delete'.
11. If it says "Delete Partition", then in the top left of the program just below 'File' there's a button for 'Apply'. Click 'Apply'.
12. A pop-up will appear asking if you want to apply the Delete partition operation. Select 'Apply Pending Operations' button.
13. Operation will finish. Press 'OK'.
14. In the middle box it will now say 'unallocated', 'unknown', and the drive size.
15. Use the left trackpad to click it, and select 'New' from the drop-down menu.
16. In the pop-up, change the 'File System' option to 'ext4' and click 'OK'.
17. Press the 'Apply' button in the top left of the program
18. A pop-up will appear. Press the 'Apply Pending Operations' button.
19. It will take about 50secs to create a new ext4 partition. After it's done, the Desktop will pop-up a Disk & Devices alert showing a new SD was inserted. Just shrink this using the down-arrow in the bottom right corner.
20. Hit 'OK' on the box in KDE Partition Manager then close it.
21. Click the 'Return to Game Mode' button on the Desktop
22. Once you're in Game Mode, eject your SD card and re-insert it.
23. Click the Steam button and go to 'Settings'
24. Scroll down to 'Storage' and click it. It should show your 'Internal Drive' and 'MicroSD Card'. The MicroSD Card should have a gold star next to it, meaning it's the default installation drive. If not, select it and hit 'X' to 'Make Default'.
25. Install a game then hit the Steam menu button and go to 'Downloads' to watch the progress, download speed, and disk usage speed.