The input to linkage is arranged with rows as observations and columns as variables, but in your example tt is a 1x7 row vector, suggesting a single observation at each of 7 variables. Instead, transpose tt to a column vector if this data represents 7 observations of a single variable, and you can then plot the dendrogram:
% your original tt variable
tt = 1.0e+03 .* [3.8334 3.9707 3.8887 2.1713 2.5616 2.3764 2.4533];
% transpose from row vector to column vector
tt = tt';
% proceed as planned
tree = linkage(tt,'average');
figure()
dendrogram(tree)