Read the error message and follow its advice to use curly-brace subscripting rather than parentheses:
data = readtable('rans1_dynamicpoints.txt');
T = data{:,1};
T2 = data{:,2};
plot (T,T2,'x');
Learn the difference: https://www.mathworks.com/help/matlab/matlab_prog/access-data-in-a-table.html
Is there a particular reason why you need to use a table? Would a simple numeric matrix suffice?:
data = readmatrix('rans1_dynamicpoints.txt');
T = data(:,1);
T2 = data(:,2);
plot (T,T2,'x');
