Hi @PrashantK270430 ,
I can understand you have enabled Now assist panel in your customer DEV instance. 

Please check relevant points as well to troubleshoot the issue -
Relevant role should be  available "now_assist_panel_user" in the user profile
Product link : https://www.servicenow.com/docs/bundle/yokohama-intelligent-experiences/page/administer/now-assist-a...

- Please clear your browser cache and cookies, sometimes  UI element not available because of  cookies and cache.

- Also check any recent customization there to hide UI element.

Please mark helpful and correct answer if it's worthy for you.