Hi Dwijayanto, welcome to the community.
I think you want to declare destination material into the fp_tank material named ‘tank’, so it should look like dest_mat = fp_tank # or their name 'tank' instead of dest_mat = ‘fp_tank’
Also, have you set that material into depleteable and set its corresponding cell volume? I think this material should exist on the openmc geometry model since openmc.deplete remake of the xml file used for depletion calculation.
But I think the experts will comment if this error came from the same problem encountered in this discussion