I think you may not need to add a new "div" but actually, you need to only insert a new paragraph as you said you only need a new text box. So, try the following code and revert the feedback:
Solution #1
headers={'Authorization': 'Bearer ' + token,'Content-Type': 'application/json' }
content_end_point='https://graph.microsoft.com/v1.0/me/onenote/notebooks/pages/{page-id}/content'
data= [{
'target':'body',
'action':'append',
'content':'<p>Hello World!</p>'
}]
result=requests.patch(endpoint,headers=headers,json=data)
print(result.text)
Solution #2
headers={'Authorization': 'Bearer ' + token,'Content-Type': 'application/json' }
content_end_point='https://graph.microsoft.com/v1.0/me/onenote/notebooks/pages/{page-id}/content'
data= [{
'target':'body',
'action':'append',
'content':'<div><p>Hello World!</p></div>'
}]
result=requests.patch(endpoint,headers=headers,json=data)
print(result.text)