My id has $5. It costs $4.99 to unlock all functions after downloading widgy, but he said that the purchase could not be completed.

If you get a message -> Your purchase could not be completed, you need to update your payment method or add a new one. 

Or add funds to your Apple ID balance as payment method: Add money to your Apple ID balance

Add money to your Apple ID balance – Apple Support (UK)

What you can buy with your Apple Gift Card or Apple ID balance

What you can buy with your Apple Gift Card or Apple Account balance – Apple Support (UK)

How to redeem your Apple Gift Card or App Store & iTunes gift card

How to redeem your Apple Gift Card or App Store & iTunes Gift Card – Apple Support (UK)

If you update your payment method or add funds to your Apple ID balance, It would take about 1 to 3 business days in order to process.

Then see -> Check your Apple ID balance

Check your Apple ID balance – Apple Support (UK)

You need the $4.99 plus any applicable taxes, which probably puts the cost over $5. You need to add funds to your Apple ID using the links posted above.