If you have R2019a or above, use readmatrix.
But, the issue is that you are mixing up passing a variable and a string. It happens that you're code returns something slightly parseable:
>> 'dtname':'dtname'
'd'
but not what you expect. Try this:
a=randi([1,7092]);
b=a+1;
columnname="D";
dt = xlsread('Final_NGA_Flatfile.xlsx',columnname+b+":"+columnname+b);
% or
dt = readmatrix('Final_NGA_Flatfile.xlsx','Range',columnname+b+":"+columnname+b);
since
>> b=101;
>> columnname="D";
>> columnname+b+":"+columnname+b
"D101:D101"
