How do I enter if I have lent money for an interest?

Loan given or money receivable is an asset. Just enter it as a manual generic asset, which is as simple as how you would enter it in a spreadsheet with 2 columns (name and value) and more metadata as details. It's the same as a term deposit in your bank account for an interest. You are basically lending your money to the bank.

Enter the asset name and the amount you lent as its value. (If they have partially paid back the principal, the value should be the current loan balance)

Click on the 4-line icon on the right side of the row to see the asset details screen.

You may add more details here (Description and notes), add documents if any.

In the Returns tab, Enter the first Cash IN for the amount you lent (That's the money you invested in this asset).

Enter Cash OUT for the interest you got. (No need to enter every month. Just make one entry for the total interest money you got for a year as on Dec 31)

Enter Cash OUT when you get the principal paid.

After you have finished the cash flow entry it will show you the IRR of this asset.

Entering cash flow to see IRR works for all kinds of assets. https://help.kubera.com/article/79-irr-of-my-investment-in-kubera

Finally when you get the whole money back, you update the current value of the asset as zero. Make a Cash OUT entry for the last payment you got. And Archive the row.