Hello Pongrapee,
I think apart from the interest condition, you need a condition for an annuity repayment with amount 50,000 - this will make sure the sum of interest and repayments are 50,000 for all periods.
If you leave the annuity amount blank, the system will calculate the annuity in a way that the sum of interest and repayments remain constant and all the nominal is repayed at the end of term.
But in both cases you will have to give the interest percentage in the interest condition.
Hope this helps!
Marie