Cannot Update Apps on MacOS App Store

Solution 1:

Please backup all your data. Example: with Time Machine or similar Software (if you not use Time Machine).

All steps are complete in itself. If any of these works you don't need the other steps.

Step 1: Appstore

Open the Appstore and choose from the Menu Store the option to show your Account. You need your AppleID password to access to your account.
If you have access to your account, click on the button reset all warnings.
Log out from the Appstore and close the app. Start Appstore again
Log in to your AppleID by using the option from the Store menu.
Test if the update process works.

Step 2: Using Terminal

Open your Terminal. Choosing from the Finder Menu Go To.. the Option Utilities. Start the Terminal.app
Type or insert the following command in your Terminal and hit Enter.
```
softwareupdate -i -a
```

If this not work or you got an error use the following command.

    sudo softwareupdate -i -a

Test whether AppStore updates work.

Step 3: The Hard Way

Reset PRAM
To Reset PRAM, restart the Mac and hold the keys CMD+Option (Alt)+P+R simultaneously during startup. Hold down the keys until you hear the second sound output.

Move the following files to the trash or on your Desktop. Close the AppStore before using the following instructions.

Click on the Finder Desktop and press the keys CMD+Shift+G.
Insert the following Line in the window and press Enter.

~/Library/Preferences/

Delete the following files (if exists)

com.apple.appstore.plist
com.apple.storeagent.plist

Restart your mac and test if works.

Step 4: Use Debug menu

Close App Store with CMD+Q
Open Terminal and insert the following Command:

defaults write com.apple.appstore ShowDebugMenu -bool true
Open App Store and choose from the Debug Menu Show download Folder
Close App Store and move the Download folder to the trash.
Restart your mac (this is important), empty the trash and try if app store works.