My model only works with a weird law of motion of capital

[jpfeifer]

See e.g.

Steady state from fsolve to calculate parameters in dynare

Use a dedicated steady state file. Within that file you can call any Matlab function and set any parameter you need. An example is the NK_baseline.mod in the Dynare examples folder.

and

I used fslove to find my SS values but there is Error massages from dynare

An example of how to call fsolve in a steady state file is