It would help if you've posted some sample data. But let's try a few variations.
If only a single payment can exist per customer then to list customers with payment, perhaps something like
SELECT c.Cust_Id
FROM user_detail c
INNER JOIN installments i ON i.Cust_Id = c.Cust_Id
WHERE c.Status = 'paid'
If multiple payments can exists and one payment will suffice then something like
SELECT c.Cust_Id
FROM user_detail c
WHERE EXISTS (SELECT 1
FROM installments i
WHERE i.Cust_Id = c.Cust_Id
c.Status = 'paid')
If all payments must be paid then perhaps
SELECT c.Cust_Id
FROM user_detail c
WHERE NOT EXISTS (SELECT 1
FROM installments i
WHERE i.Cust_Id = c.Cust_Id
c.Status = 'not paid')
And if you want the total count of customers you can change the SELECT clause in previous examples to
SELECT COUNT(*)
...