It is because you have to fast the 6 days of Shawwal after you have completed Ramadan, thus, you can’t get the reward of fasting shawwal while your Ramadan is still incomplete.

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The normal fasting that was prescribed by the Prophet salla Allahu alaihi wa sallam is to fast alternate days! This is the maximum in normal conditions. However, if a person is afraid that he would inevitably fall in sin, he may fast continuously as this is a precaution

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