First, don't name a variable timetable. That already has a meaning in MATLAB.
Second, at least two rows of your timetable (which I'm going to call TT) have the same value for their RowTimes.
X = TT.Properties.RowTimes;
any(X(1:end-1) == X(2:end))
Suppose I had two vectors:
v1 = [1:5 5:10];
v2 = 1:numel(v1);
plot(v1, v2, 'o-')
If I wanted to interpolate THE value of v2 when v1 = 5, what should that ONE value be? 5, 6, 5.5, NaN (since it's not unique), etc.?
How you fix that problem will depend on your specific data and the problem you're trying to solve.