batxpres, we are here to help and make sure that you are able to enjoy all the features that come with your phone. Just to be sure we can verify the steps to take a screenshot, tell us more, what type of phone do you have? With most devices, you are able to take a screenshot by pressing and holding both the Power and Home buttons at the same time.
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How many times have you tried this? Could be the volume button is being activated prior to and not simultaneously. It may be tricky.