Unable to use guided access

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

When done, read this support article carefully:--> Use Guided Access with iPhone, iPad, and iPod touch - Apple Support

If the issue still persists, contact Apple Support directly, using this link:--> Get Support, or 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.

My pleasure :)

Giulio