See these can't-update/download possibilities. They are the only documented explanations I know of for your problem. I know that several of these possibilities don't seem likely in your case.
'Your account has been disabled in the App Store and iTunes' https://support.apple.com/en-us/HT208856. This link says: "There might be a problem with the payment method that you have on file. When you see the message, don't try to edit any of your account information. Instead, contact Apple Support for help.”
Try using the Apple recommended support link above first. But, this site is down/not fully functional very often. Use the support options I give below if necessary.
You can use Apple Support Twitter https://twitter.com/AppleSupport There may be an hour or so wait.
And, if necessary, go here to contact iTunes worldwide support via text or chat https://support.apple.com/choose-country-region/itunes After choosing your country, see the heading, Tell us how we can help.
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If you can’t download or update apps on your iPhone or other Apple device https://support.apple.com/en-us/HT207165
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If you can't connect to the App Store, iTunes Store, or other Apple services https://support.apple.com/en-us/HT201400. There have been quite a few Apple server connection problems lately. Among other things, in the above document, see Check Apple’s System Status page. You can check there to see if the needed server is available when you need to use one.
Also, below are the alert messages given in the document. But, receiving others is a possibility. Use the steps in the above document if you see one of these error messages or a somewhat similar message.
· "Cannot connect to the iTunes Store. An unknown error has occurred."
· "We could not complete your iTunes Store request. The iTunes Store is temporarily unavailable. Please try again later."
· "Could not connect to the iTunes Store. Make sure your network connection is active and try again."
· "We could not complete your iTunes Store request. The network connection could not be established. There was an error in the iTunes Store. Please try again later."
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What to do when the Mac App Store shows a different Apple ID for software updates https://www.macworld.com/article/3267709/mac-app-store-apple-id.html. This document refers to a prior owner (use the discussion in it this is your situation), but is also applies to cases where a current owner's old Apple ID is the problem. The discussion below relates to the second possibility.
Basically, If an app is purchased/downloaded using one Apple ID, then you can't update it with a second Apple ID. The app is inextricably linked to the first Apple ID. You'll have to delete if from the first Apple ID and purchase/download it again with the second Apple ID unless you set up family sharing (see below).
You can set up family sharing between the IDs to share purchases made with them. See:
Family Sharing https://support.apple.com/en-us/HT201060
Set up Family Sharing https://support.apple.com/en-us/HT201088
