Hello there,
The steps you need to do,
Solution 1:
1-) Right click on Steam Client while on Desktop
2-) Select Run as administrator from the context menu
and try again.
Solution 2:
1-) Launch your Steam client while on the desktop
2-) Once the Steam client opens, click on Steam in the top left and select Settings from the context menu.
3-) When the Settings page opens, click on the Downloads tab on the left
4-) As soon as you click on Downloads, you can see the Clear Download Cache button on the right.
5-) Once you click the button, you will see a warning message that steam will restart and you will need to log in again.
6-) Click OK to continue
If these steps don't work, try deleting and reinstalling Steam.
I hope it will be of use to you.
King Regards,
Nightmaree.