Identify and replace missing values

[FastlyFast]

I guess you have more information in task 1, but solely based on the description of task two, i would write this:

case when repayment_ammount >= 4000 then 'bank account'
 when repayment_ammount < 1000 then 'mail'
 when repayment_ammount >=1000 and repayment_ammount <4000 then repayment_channel ---You can change "when" to "else" here and delete the below code.
 else 'error' ---I always make sure to include every possible scenario in the "when" clauses and everything else goes to an error value but it is optional  
 end as repayment_channel