You added an initval block to your mod-file that tries to access yss_1, which has not been defined. If you are using a correct steady state file that produces the correct analytical steady state, you can simple delete the initval block.
Thanks Jpfeifer for your reply,
I corrected this but the problem remain. Can you, please, verify for me if the steady state file is correctly specified in terms of macro-processor?
Best,
You must do that yourself. You can see the resulting steady state file as a matlab m-file in your folder after running the preprocessor. You can use F9 in Matlab to execute selected parts of the code. Note that your problem does not stem from the steady state file but from the initval-block. To see what the preprocessor does, use the