An interest-only period postpones repaying the principal. If the contract requires paying interest during that time, the business still needs cash and the debt balance stays the same.
An example with numbers
On a $60,000 loan at 6% nominal a year, six months paying only interest means $300 a month with a flat monthly calculation. You will have paid $1,800 and still owe $60,000 when the period ends.
Try it with your numbers
Draw two stages in your forecast: interest-only and repayment. If the final maturity does not change, check how many months you will have to repay the principal in.
This calculator is on its way. In the meantime, you can do the calculation on paper or in a spreadsheet.
Example figures for learning purposes. They are not a recommendation and do not replace professional advice.