Help to update my SQL query

In the case of whether the date exists or a valid date: the text calculates the period between two dates per month and days and multiplies it by the amount of compensation in the number of person who benefit from it
Example: Number of person 2, compensation amount 4000 DZD, the period between 01/04/2020 to 2/18/2020 (1 month and 14 days)
(((4000 DZD * 14 days) / 30) + (4000 DZD * 1 month)) * 2 Number of people = 11733.33 DZD
problem in query
If the file has one compensation, it will calculate, and if the file has a set of compensation, an error is displayed

Msg 512, Level 16, State 1, Line 4
Subquery returned more than 1 value. This is not permitted when the subquery follows =,! =, <, <=,>,> = Or when the subquery is used as an expression.
Msg 512, Level 16, State 1, Line 14
Subquery returned more than 1 value. This is not permitted when the subquery follows =,! =, <, <=,>,> = Or when the subquery is used as an expression.

the question:
How can I show all the compensation and the corresponding accounts for each file separately so that if I change the file number for example "20/0003"

declare @days int
declare @month int
declare @Total dec(18,2)
if (select isdate(date_debut1) from Indemnite  where id_aff='20/0002') =1 begin
select @month = DateDiff(month, date_debut1, date_fin1)  from indemnite 
select @days = DateDiff(day, DateAdd(month, DateDiff(month, date_debut1, date_fin1), date_debut1), date_fin1) from indemnite where id_aff='20/0002'
if @days < 0 begin
select @month=@month-1  from indemnite 
select  @days = DateDiff(day, DateAdd(month, @month, date_debut1), date_fin1) from indemnite  where id_aff='20/0002'
end
select @month=@month *1% 12 
select @Total  =(((montant_dette1 * @days)/30)+(montant_dette1 *@month))* n_personne1 from Indemnite  where id_aff='20/0002'
end 
if (select isdate(date_debut1)from Indemnite where id_aff='20/0002') =0 begin
select @Total  =(montant_dette1 * n_personne1) from Indemnite where id_aff='20/0002'
end 
select Indemnite1, n_personne1, date_debut1, date_fin1, montant_dette1,@Total as Total  FROM Indemnite  where id_aff='20/0002'