Hello @Kylian_15,
Please run Windows Update troubleshooter from Settings app > Update & security > Troubleshoot > Additional troubleshooters. See if it works.
If that doesn't work, try this:
Open administrative Command Prompt and type following commands one-by-one followed by Enter key.
net stop wuauserv
net stop cryptSvc
net stop bits
net stop msiserver
Ren C:\Windows\SoftwareDistribution SoftwareDistribution.old
Ren C:\Windows\System32\catroot2 Catroot2.old
net start wuauserv
net start cryptSvc
net start bits
net start msiserver
Close Command Prompt and see if Windows Updates works then.
If you still unable to install 21H1, I recommend you to upgrade to Version 21H1 via ISO file.
Refer this step-by-step guide:
https://www.kapilarya.com/how-to-upgrade-to-windows-10-using-iso-file
Let us know if this helps!
Note: Included link in this reply refers to blog post by a trusted Microsoft MVP.
