1.
What happens when you start Music -> Click on iTunes -> sidebar click iTunes Store -> go to Account (top bar) -> View Account -> then Sign in?
See also the following Apple articles:
About the 'Trust This Computer' alert on your iPhone, iPad, or iPod touch
About the 'Trust This Computer' alert on your iPhone, iPad, or iPod touch - Apple Support
Authorize or deauthorize your computer for iTunes Store purchases
Authorize or deauthorize your computer for iTunes Store purchases - Apple Support
To add a new device, see -> Check your Apple ID device list to see where you're signed in
Check your Apple ID device list to find where you're signed in - Apple Support
2.
Make sure your Windows PC is up to date.
Update to the latest version of iTunes -> Update to the latest version of iTunes for Windows 10 – Apple Support (UK)
See also -> Check your third-party security software.
Check the Wi-Fi connection.
Turn Off Hotspot.
Turn Off or remove VPN if necessary.
Disable all AV & Firewalls temporarily.
To process the changes, restart your PC.
Then try again.