Hi, I am Dave, I will help you with this.
Open the old Control Panel (not the settings app)
At the top of the old Control Panel, set view to icons
Open Power Options
On the left, click 'Choose what the power buttons do'
At the top right, click 'Change settings that are currently unavailable'
Under 'Shutdown settings' disable 'Turn on fast startup'
Save that and close Control Panel
Restart (not shut down) your PC to apply that change.
Then test if you an shut your PC down.