I do all that you say
I still have the same error
if I delete the create method I don't have error
but when I add it and run for multi record I faced the error

Hi ,
Can you please try with the below code.

# Server Action
record id="generate_salary" model="ir.actions.server">
field name="name">calculate salary</field>
field name="model_id" ref="hr_salary.model_hr_salary_details"/>
field name="binding_model_id" ref="hr_salary.model_hr_salary_details"/>
field name="binding_view_types">list</field>
field name="state">code</field>
field name="code">
if records:
records.calculate_salary()
/field>
/record>

# Let the system take the default api
def calculate_salary(self):
salary_mnth_obj = self.env['hr.salary.months']
for rec in self:
salary_mnth_obj.create({
'name': ('salary for') + " " + str(rec.employee1_name_id.name),
'employee1_name': rec. employee1_name_id.name,
'salary_id': rec.id ,
'employee_number': rec.employee_number_id,
'employee_job_title': rec. employee_job_title,
'employee_salary': rec . employee_salary,
'hawafez_salary': rec. hawafez_salary,
'cooperation_box': rec . cooperation_box ,
})
return True

Hope it will help you.

Hi Ali Ammar,
Please check the updated answer. I hope it will help you.

raise exception.with_traceback(None) from new_cause
ValueError: <class 'ValueError'>: "Expected singleton: hr.salary.details(1, 2, 3)" while evaluating
'if records:\n records.calculate_salary()'

after I run your code

Then you must be check the code for the create and compute methods of the model "hr.salary.months" and the api decorators for those methods.
I hope it will help you.

the problem in create method and I don't know how to solve

Please share the create method.
Thanks

I didn't write any thing else over the above code
create method still like itself

Ok , then try to add a create method in the model and check the result. Hope it will help you.

What should I write in this method?

I solve singleton error
thanx
but now it's create only the first record in my selected records as much i select records

Please print the self into your code and check if it takes the multiple records or not.
User active_ids from the context instead of the self.
Hope it will help you