The contact model template project appears to be broken at the moment.
I uploaded a PluginFiles archive at https://itasca.sharefile.com/d-s139c705205cb402abc99127518be5944.
This archive contains an example contact model under PluginFiles\contactmodel\example.
Please use this example as a starting point to implement your custom contact model, and do not hesitate to come back to me for further assistance.
I am sorry for the inconvenience.
I compiled a contact model with the PluginFiles archive you shared, but the stress function getOldStress cannot access the stress value.
How can I solve this problem?