"What im trying to do is to add all the amplitudes off column 1 for a specific frequency range"
This is a different question from what you originally asked, so would have been better started as a new question.
Anyway, for this you'd use discretize to assign unique numbers to each frequency band and splitapply to sum all rows that belong to a given band:
frequencybands = [0, 24, 51.5, 80, Inf]; %Creates 4 frequency bands: [0, 24), [24, 51.5), [51.5, 80), [80, Inf]
bandnumber = discretize(K(:, 2), frequencybands); %assign unique band number to members of a band
bandamplitude = splitapply(@sum, K(:, 1), bandnumber); %sum amplitudes that belong to the same band
Another option would be to convert your matrix to a table and then use groupsummary. A table also has the advantage of making it clearer what each column represent.
ampfreq = array2table(K, 'VariableNames', {'Amplitude', 'Frequency'});
frequencybands = [0, 24, 51.5, 80, Inf]; %Creates 4 frequency bands: [0, 24), [24, 51.5), [51.5, 80), [80, Inf]
bandamplitude = groupsummary(ampfreq, 'Frequency', freqbands, 'sum', 'Amplitude'); %sum amplitude according to frequency bands
