You can start by going to Settings > Apps & the Play Store, then Storage & Clear both Cache and Data for the Play Store & Reboot.
See if you can now download / update your apps?
If that doesn't work, you could try Booting into Recovery and wipe Cache, but that's not likely to fix the issue if Wiping Cache & Data for the Play Store didn't work.
If I was experiencing the same issue, at this point I would backup my phone with Smart Switch & Factory Reset.
I actually tried those steps mentioned above. Cleared cache for, Play store Play services Download manager Update manager System cache (from recovery)
Removed and added google account
Uninstall updates for Google play store and again setting whole play store.
Well factory reset is last option I believe. It will be difficult to take backup for me because I have like 180gb data on my phone.
I can do reset but this is just a work around. I need to what caused my system to be in such state so that i can be aware of such possible issue in future.
To try and determine the cause, you could try to get a log while trying to update / install apps from the Play Store. That should show the error.
If you know what you're doing, then you could do it yourself.
If you're unsure, then you could use the "Get Help" in the Samsung Members app & Scroll down to Feedback / Error Reports & it should capture the error & send it to Samsung. Hopefully they will get back to you with the explanation?
UPDATE : I am somehow able to update instagram, whatsapp from play store but not other applications. I can only update though. Further more, I can download and install apps from galaxy store but the same app I can't download from play store. Really strange behaviour. I am still waiting for the support team to check logs and reply.
Depending on what they reply I will think of factory resetting the phone.
