Issues running PEST [error with .par file]

[Svea Bertolatus]

Hi all, 

I have a model with an unknown parameter Evapotranspiration that I want to solve for using PEST. I am only using the PEST withing GMS Modflow 9.0.5, no additional Model Muse. 

The model works in transient condition (without adding the PEST to it). When adding the unknown parameter evapotranspiration and then the Parameter Estimation, then it does not converge. I have been trying many things, have also re-installed GMS, but the error remains: Cannot open file [name].par .

Is there anyone who has experienced a similar error and knows how to solve it? 

Best wishes and thanks in advance!

Svea