I am trying to power off my iPhone 11 and the screen with the power off slider is unresponsive- how can I unlock this screen?

Try a "forced" restart of your iPhone, using these instructions:--> Force restart iPhone – Apple Support

If the issue persists, read also this support article:--> If the screen isn't working on your iPhone or iPad - Apple Support

If you still get no help, I suggest that you make an appointment (*) and have your device inspected at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> Apple – Location Selector. After checking it out, they'll give you all the available options.

Regards

Giulio

(*) You can do it:

• Online:--> Genius Bar Reservation and Apple Support Options - Apple;
• Contacting a local Apple Store or AASP among the list provided above;
• Using the "Apple Store" app, available and downloadable from AppStore.