To diagnose and fix the issue, perform checks in following order
Ensure EPT code is submitted to server. You can check BLOB column(it contains ept zip) in REF_EPT table for respective ept.
Ensure action is invoked using user context which has access to EPT. i.e. the enterprise name on used role should match with enterprise name of EPT or EPT is available for the role enterprise. Note: if this action is not originating action and is being invoked by some code then check the user context used in the code while invoking the action.
Ensure Custom model name used while executing action or the model name on the model object matches with custom model name in EPT under which callback is defined. In EPT, standard custom model (e.g. Standard Partner) are available by default. If custom model name used while executing action is not the Standard model then you can apply your changes in Standard model in EPT and rename the Standard .modelDef file(assuming the .modelDef is newly created) with the used custom model name and replace all Standard model name occurrences in .modelDef file. After this change you can re-open EPT in studio and verify the updates. Once verified you need to submit EPT again.