before estimating a model, you have to be sure that your model has (at least) one stable equilibrium for a given set of parameters. You have that error because Dynare starts the maximization of the likelihood with an initial combination of parameter values (the value you fixed for the parameters you fixed and the mean of the prior distribution for the parameters you are going to estimate) which have to solve the model, i.e. the have to allow for an equilibrium to exist (and be stable). In you case you have picked values which give you an unstable equilibrium.