Why is my phone not letting me update my apps but buy other apps ?

Check your purchases history to see if the apps were downloaded using the current ID.

Purchase History Review. 

This most likely is your problem. See 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, but is also applies to cases where a current owner's old Apple ID is the problem.

Basically, If an app is purchased/downloaded using one Apple ID, then you can't update it with another Apple ID. The app is inextricably linked to the first Apple ID. You'll have to delete it and repurchase/download it again with the second Apple ID.