Here are some sentences I found:
Chances may only knock the door once so if you want to accept that as a challenge you must take every step seriously.
This description uses no comma at all and just one full stop. However, there are many sentences included in the description, and each one must end with a full stop. Also, there is an introductory words used, which is "So", so commas should also be used to separate it from the rest of the sentence.
This is how I would punctuate the description to improve text clarity:
Chances may only knock the door once. so, if you want to accept that as a challenge, you must take every step seriously.