There are so many problems with your Windows 10 Store. You can not open the Windows 10 store, you cannot download applications with the Windows 10 store. So I think you can try to do these things:
1. Try to logout the Windows 10 Store and login again. Here you should log out your Windows 10 account and then login it.
2. Update your Windows 10. The new version of Windows 10 may fix bugs and you can use it normally.
3. Run Microsoft Windows Store Apps Troubleshooter. Here is the steps: Start > Settings > Update & Security >Troubleshoot > Windows Store Apps > Run the Troubleshooter.
4. Reset your Windows 10 Store Cache. And you said you had done it. But try again after the above three steps. The...