Hey Loh
Opportunity.acct.type refers to the prospect acct here.
your err is basically stating that when the opportunity with prospecting stage and prospect was inserted,…the task was not created successfully.
so check that the process builder is active, the it is set to when record is created and edited and then check that it check the opportunity and opportunity acct type condition and if yes… create a record>task> give proper subject headline and it shud work.
you can also debug the Process builder is still issue persists