I can't tune my guitar correctly

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I own a Stagg G300 electric guitar and, even tuning it with an electronic tuner, there is a difference in pitch at the 12th fret. I checked the neck curvature: pressing down the E string on the first and the last fret, there is a space of a credit card thickness between the string and the neck... Is the neck set in a wrong way? Then I set the bridge but there is a difference in pitch (about 15-20 % of a semitone) What can I do to tune correctly my guitar?