Translation:Please call an ambulance for me.
The sentence is a single request to call for an ambulance. It is not 2 separate requests/sentence to ask for help and call the ambulance. Your sentence would translate to "请帮我，帮我叫护车。"
"Please call an ambulance for me" is a correct translation, as 帮我叫救护车doesn't specifically mean that we will call together but that I may need you to call on my behalf.
Just because 帮 means help doesn't mean a word for word translation is accurate. The request is for someone to call an ambulance on the speaker's behalf (thereby helping the speaker). So "please call an ambulance for me" is correct.
Can you call and please call do not make any difference... Duo is extremely pickish and does not know much English but only chinglish...