1
Your Quick Access folder is probably corrupt, the best option is to delete the content of Quick Access, and then re-pin the folders you need there.
Click your Start Button, then just type cmd and on the resulting menu, right click Command Prompt and select 'Run as Administrator'
Paste this command into Command Prompt and press Enter:
del /F /Q %APPDATA%\Microsoft\Windows\Recent\*
Paste this command into Command Prompt and press Enter:
del /F /Q %APPDATA%\Microsoft\Windows\Recent\AutomaticDestinations\*
Paste this command into Command Prompt and press Enter:
del /F /Q %APPDATA%\Microsoft\Windows\Recent\CustomDestinations\*
Then, close Command Prompt.
2
Then open File Explorer.
Click the 3 dots on the top menu and select properties.
On the resulting dialog, click the 'Restore Defaults' button.
Select the View tab and click the 'Restore Defaults' button and the 'Reset Folders' button.
Restart (not shut down) your PC.
Then try adding new folders/files to Quick Access.