Are you trying to insert all of the invoice items for all students with an invoice date of '2019-09-05'? If you use a single query to get all the students you are interested in creating invoice items for, then in the end you would have a total of two insert statements. The one below, then a similar one for the 'Tuition_Service_Fee'. Also, using table joins instead of sub queries in my example.

INSERT INTO [dbo].[InvoiceItem] (invoice_item_name, amount, invoice_id)
select "Tuition", Grade.tuition_amount, invoice.invoice_id
from Student
join invoice on student.student_id = invoice.student_id
join grade on student.grade_id = grade.grade_id
where student.invoice_date = '2019-09-05'
--if you really want to specifiy the first names
and student.first_name in ('Murdoch', 'Hartwell')

Are you trying to insert all of the invoice items for all students with an invoice date of '2019-09-05'? If you use a single query to get all the students you are interested in creating invoice items for, then in the end you would have a total of two insert statements. The one below, then a similar one for the 'Tuition_Service_Fee'. Also, using table joins instead of sub queries is preferred in my opinion - more readable and can be better performing. But definitely, use variables instead of hardcoded values will allow the code to be reused whether you just call the script or create a stored procedure.

declare @invoiceDtm datetime

INSERT INTO [dbo].[InvoiceItem] (invoice_item_name, amount, invoice_id)
select "Tuition", Grade.tuition_amount, invoice.invoice_id
from Student
join invoice on student.student_id = invoice.student_id
join grade on student.grade_id = grade.grade_id
where student.invoice_date = @invoiceDtm
--if you really want to specifiy the first names
and student.first_name in ('Murdoch', 'Hartwell')

Is this what you after?

INSERT INTO [dbo].[InvoiceItem] (invoice_item_name, amount, invoice_id)
select "Tuition", Grade.tuition_amount, invoice.invoice_id
from Student
join invoice on student.student_id = invoice.student_id
join grade on student.grade_id = grade.grade_id
where student.invoice_date = '2019-09-05'
--if you really want to specifiy the first names
and student.first_name in ('Murdoch', 'Hartwell')

Are you trying to insert all of the invoice items for all students with an invoice date of '2019-09-05'? If you use a single query to get all the students you are interested in creating invoice items for, then in the end you would have a total of two insert statements. The one below, then a similar one for the 'Tuition_Service_Fee'. Also, using table joins instead of sub queries in my example.

INSERT INTO [dbo].[InvoiceItem] (invoice_item_name, amount, invoice_id)
select "Tuition", Grade.tuition_amount, invoice.invoice_id
from Student
join invoice on student.student_id = invoice.student_id
join grade on student.grade_id = grade.grade_id
where student.invoice_date = '2019-09-05'
--if you really want to specifiy the first names
and student.first_name in ('Murdoch', 'Hartwell')