Can not purchase in-App

I just want to pay via gift card.

Will it work if I buy and send one more gift card to my Apple ID ?

Yes sure, see -> 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)

See -> What type of gift card do I have?

What type of gift card do I have? – Apple Support (UK)

Actually, you can buy an Apple gift card at many places, it depends of course on where you live.

At an Apple Store or an Apple Authorized Reseller in your region certainly, but also at a good bookstore, supermarket and/or drugstore.

FYI: You can't redeem Apple Gift Cards or App Store & iTunes Gift Cards outside the country or region of purchase. For example, if you bought an App Store & iTunes Gift Card in France, you can't redeem it in the App Store in the United States.

See the following Apple articles:

Check your Apple ID balance

Check your Apple ID balance – Apple Support (UK)

Add money to your Apple ID balance

Add money to your Apple ID balance - Apple Support (IN)

But when I make a US $1.99 purchase in-App, it shows "can not complete".

That means that you have to -> Add money to your Apple ID balance

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

Contact Apple Support ->  Get Support (apple.com)