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

Loan given or money receivable is an asset. It's the same as a term deposit in your bank account for an interest. You are basically lending your money to the bank.

Just add it as a manual asset entry, just like how you would in a spreadsheet.

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 balance amount payable)

Update the value as you go.

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

If there's a regular payment coming to you, you may use the Cruise Control feature to automatically update the value with an interest rate every month and then reduce the balance using regular payment amount that comes to you.

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 the pay outs you get as Cash OUT entries.

This will show you the rate of returns (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.