It failed, because the colon operator does not accept a random increment. Anyway, randn(1:5) does not generate a random number between 1 and 5 anyway. I'd suggest you need to read the getting started tutorials.
Doing what you have asked is not trivial however. One approach would use Poisson arrivals. But I recall that would imply an exponential interarrival time. And you are asking for a uniformly distributed interarrival time.
Simplest would be to use Roger Stafford's randfixedsum, available for free download from the File Exchange.
If the spacing is to be from 1 to 5, then the average spacing is 3. Over the interval [1,1600], that means you would expect
So you want to find a set of 533 random numbers, uniformly distributed on the interval [1,5] that sum to 1599.
delta = randfixedsum(533,1,1599,1,5)';
tsyn = cumsum([1,delta]);
As you can see, this set has the desired properties. It runs from 1 to 1600.
As well, it is uniformly distributed in terms of the inter-arrivals. (Well approximately so. With a larger sample size, it will be closer to uniform.)
You can find randfixedsum here: