If you are using the Block editor and have the jetpack plugin enabled you can generate a QR code for your post.
You can generate the QR code after you have published a post or page.
- Click the Jetpack icon at the topright of the post editor.
- Click QR Code in the pannel that appears.
- Press the Get QR Code Button, your QR code appears and you can right click to download.

Remember you need to have activated the Jetpack plugin and be using the Block Editor.