In Windows 8 or later, you requires a strong network connection if you’re signing in for the first time using Microsoft Account. You also need a non-intermittent web connection when you’re using Windows Store. Many times users have fluctuating Internet connection and they start complaining that the Windows Store doesn’t works at all. Users may also came around following error while signing in using Microsoft Account on their system:
We Couldn’t Sign You In

We too faced this issue still after having a HSPA connection with speed of around 21 MBPS. This was enough good speed to go, but unfortunately system doesn’t allowed us to sign in. Hence it clearly seems like network is not the culprit in our case, and we need to try following fix to solve this issue:
1. Press
Windows Key + R combination and put
inetcpl.cpl and hit Enter to open Internet Properties.

2. Next, in the Internet Properties windows, click on the Connections tab and click
LAN Settingsbutton then:

3. Moving on, in the LAN Settings box shown below, uncheck the
Use a proxy server for your LAN option. Click
OK.

You could also try the Windows Apps Troubleshooter. It would automatically find out and fix issues if any.
Finally, open administrative Command Prompt and type
wsreset.exe and hit Enter. It will clear the Windows Store cache on your system. Check out now if you can sign in, you should now be able to do so easily.
twc