Error using fprintf Invalid file identifier. Use fopen to generate a valid file identifier. Function help

When i run the function and enter the following outputs, i get this error message:

This program accepts a vector of masses and calculates the energy for
  them. It displays a linear and three logarithmic plots and provides a
  table of values for mass entered and resulting energy from the row vector
  entered.
  
Enter your choice   1.Metric  2.US customary units : 1
Enter your choice     1.Vector  2.Scaler : 2
Please enter mass in Kg=10
E =
   8.9874e+17

Error using fprintf
Invalid file identifier.  Use fopen to generate a valid file identifier.
Error in energy (line 17)
    fprintf(E)

Here is the full function code:

function[E]=energy
% This program accepts a vector of masses and calculates the energy for
% them. It displays a linear and three logarithmic plots and provides a
% table of values for mass entered and resulting energy from the row vector
% entered.
%
clc
close all
help energy
c=2.9979e8;
unt_choice = input('Enter your choice   1.Metric  2.US customary units : ');
if unt_choice == 1
  vctr = input('Enter your choice     1.Vector  2.Scaler : ');
  if vctr == 2
    m=input('Please enter mass in Kg=');
    E=m*c^2
    fprintf(E)
  else  
    ln = input('Enter the length of the vector : ');
    m = zeros(1,ln);
    q = 1
    fprintf('Start Entering your vector of masses in Kg')
    while q<ln
      m(q) = input('...');
      q=q+1;
    end
    E=m.*c^2;
    subplot(2,2,1)
    plot(m,E,'b-p')
    xlabel('MASS (kg)')
    ylabel('ENERGY (J)')
    title('E vs m-NORMAL')
    grid
    subplot(2,2,2)
    semilogx(m,E,'m-p')
    xlabel('MASS (kg) IN LOG SCALE')
    ylabel('ENERGY (J)')
    title('E vs m-SEMILOGX')
    grid
    subplot(2,2,3)
    semilogy(m,E,'r-p')
    xlabel('MASS (kg)')
    ylabel('ENERGY (J) IN LOG SCALE')
    title('E vs m-SEMILOGY')
    grid
    subplot(2,2,4)
    loglog(m,E,'g-p')
    xlabel('MASS (kg) IN LOG SCALE')
    ylabel('ENERGY (J) IN LOG SCALE')
    title('E vs m-LOGLOG')
    grid
    filename = 'energy.xlsx';
    A = {'Mass (kg)';'Energy (J)'};
    xlswrite('energy.xlsx', A, 1, 'A1');
    xlswrite('energy.xlsx', [m; E], 1, 'B1');
  end
else
  vctr = input('Enter your choice     1.Vector 2.Scaler : ');
  if vctr == 2
    m=input('Please enter mass in slug=');
    E=m*14.5939*c^2
    fprintf(E)
  else  
    ln = input('Enter the length of the vector : ');
    m = zeros(1,ln);
    q = 1
    fprintf('Start Entering your vector of masses in slug')
    while q<ln
      m(q) = input('...');
      q=q+1;
    end
    m = m.*14.5939
    E=m.*c^2;
    subplot(2,2,1)
    plot(m,E,'b-p')
    xlabel('MASS (kg)')
    ylabel('ENERGY (J)')
    title('E vs m-NORMAL')
    grid
    subplot(2,2,2)
    semilogx(m,E,'m-p')
    xlabel('MASS (kg) IN LOG SCALE')
    ylabel('ENERGY (J)')
    title('E vs m-SEMILOGX')
    grid
    subplot(2,2,3)
    semilogy(m,E,'r-p')
    xlabel('MASS (kg)')
    ylabel('ENERGY (J) IN LOG SCALE')
    title('E vs m-SEMILOGY')
    grid
    subplot(2,2,4)
    loglog(m,E,'g-p')
    xlabel('MASS (kg) IN LOG SCALE')
    ylabel('ENERGY (J) IN LOG SCALE')
    title('E vs m-LOGLOG')
    grid
    filename = 'energy.xlsx';
    A = {'Mass (kg)';'Energy (J)'};
    xlswrite('energy.xlsx', A, 1, 'A1');
    xlswrite('energy.xlsx', [m; E], 1, 'B1');
  end
 end

Answer 1

[Star Strider]

Try this:

fprintf('E = %23.15E\n',E)

Choose the format you want.

Answer 2

[Asad (Mehrzad) Khoddam]

Because E is a numeric variable, it conot be used directly in your case.

Use E with some formatting:

fprintf('%g\n',E)