A matrix of parameters in Stan Integral

Yes, it was just a constant that I chose.

Update: I ran the code on a larger dataset and received the warning messages

Chain 1: Rejecting initial value:
Chain 1: Log probability evaluates to log(0), i.e. negative infinity.

I think I receive these warning messages because I chose parameter values that would ensure that the units failed within a reasonable amount of time. This means that some of the u_obs and u_cens values were blowing up.

I simulated more data using parameter values that do not have the above property and I was able to recover the true parameter values.

This time I think the warning messages

2: Bulk Effective Samples Size (ESS) is too low, indicating posterior means and medians may be unreliable.
Running the chains for more iterations may help. See
3: Tail Effective Samples Size (ESS) is too low, indicating posterior variances and tail quantiles may be unreliable.

are nothing serious, and should go away when I run the chain(s) for longer.

Update: Running the chain(s) for longer did remove these error messages.