How to enable while 1 loop in matlab?

First, you can't call the if statement if weight and height conditions have not been defined yet with the prompt. So you should call input() before if:

Weight = input('Please Enter Your Weight In kg: ');
Height = input('Please Enter Your Height In m : ');
if (Weight > 0 && Height > 0)

Now, you can run your code. Your problem is thebreak. This is how you defined your code:

while loop 1 (forever)
    prompt asking for weight and height
    if weight & height > 0 (positive)
        (...some other calculations and if)
    break

If you check this pseudo-code, you are telling your code that if weight and height are positive, break the while loop, therefore it stops. The right condition should be:

if weight & height > 0 (positive)
    run...
else (not positive)
    break

This is your code fixed:

while 1

    Weight = input('Please Enter Your Weight In kg: ');
    Height = input('Please Enter Your Height In m : ');
    if (Weight > 0 && Height > 0)
        
        BMI = Weight/(Height^2);
    
        if (BMI<=18.5)
        disp(['Health condition: THIN. Your BMI is ', num2str(BMI)]);
        
    
        elseif (BMI>=18.6) && (BMI<=24.9)
        disp(['Health condition: HEALTHY. Your BMI is ', num2str(BMI)]);
        
    
        elseif (BMI<=25) && (BMI<=29.9)
        disp(['Health condition: OVERWEIGHT. Your BMI is ', num2str(BMI)]);
        
    
        else (BMI>=30)
        disp(['Health condition: OBESE. Your BMI is ', num2str(BMI)]);
        
        end
        
    else
        break
    end
end

Try to take always care of the identation. Your first if and its final end wasn't equally.