It seems like there are some issues with the payment, so I will continue to help you here.
If the above method doesn't work, then please do the following
Solution 1 :
Close App Store. Then in Finder, hold down Option key on your keyboard, click on Go menu at the top bar and select Library. This will open Library folder. Then open Preferences folder and trash the following files
Then shut down your Mac
Restart and immediately hold down Command-Option-P-R keys at the same time
Hold the keys down until the computer restarts and you hear the startup sound for two or three times.
Release the keys and let the Mac boot up. Try App Store updates.
If it still not working, then try this solution.
Download App Cleaner application from here --> https://freemacsoft.net/appcleaner/
Open App Cleaner and load the App Store app in it. It will App Store and all it is related files in that window. So uncheck App Store app, but keep other files of App Store checked. Then click Delete button and restart your Mac. Then try to download updates.
Please let me know if that didn't work, so we can continue troubleshooting.