StatMate won't accept zero as the proportion of event-free (surviving) subjects at the end of the study.

 StatMate uses standard methods for computing sample size and power. These methods are clearly explained in Machin (see link below). 

If StatMate let you enter 0.0 as the proportion of event-free subjects at the end of the study, the equations would lead to math error. Depending on how the equations are written, StatMate would have to divide by zero or compute the logarithm of zero, and both are mathematically impossible. So instead, we simply don't let you enter zero.

Avoid the temptation to enter a very small fraction instead of zero. All the calculations work on ratios, so you'll get different results if you enter 0.01 and 0.001 and 0.0001. 

Workaround: Ignore the instructions to estimate the proportion event-free at the end of the study.  Instead, use an earlier time point, and interpret all the results in terms of that time point.