Dear iStore IT Team, I can’t update macOS or use the App Store. Can you assist with troubleshooting? Thank you,

Seems fairly obvious to me, but maybe I'm missing something.

In the second screenshot you're trying to install the 10.15.7 update, but that requires an existing 10.15.x installation, so I'm guessing your current machine isn't running 10.15, right? Therefore the install can't run.

In the first screenshot you're trying to install Mac OS, but there's no suitable disk available.

A lot of this will hinge on the OS version currently installed on your machine. What version are you running right now?

Use this link and choose macOS 10.15 Catalina

 How to get old versions of macOS

It will download the Full and Compete version of macOS 10.15.7 Catalina

Once downloaded, look in the Applications folder of the computer

There should be a new application called " Install Catalina app "

When ready, launch it just like ant other application

Follow the instructions

The computer may reboot several time before the computer is upGraded to macOS 15 Catalina

Always make a Time Machine Backup  before doing any upGrades

mandla129 wrote:

The link you sent me on the MacOS Catalina I've tried that option before and I've attached the results as follows

https://discussions.apple.com/content/attachment/36aecf96-46dc-4b82-83e3-b824aec86547

https://discussions.apple.com/content/attachment/c8a21c39-e340-48bc-804c-f3d60c14b232

https://discussions.apple.com/content/attachment/28360f49-7268-42c1-bef1-b0a57bbca6cc

Right now there are 3 different Contributors in your question plus yourself @ mandla129

To get the replies to your question in chronological order by Date 

Click on your User Name  >>   >> " Edit Profile and Preferences "

Scroll way down and look for >> >> " Other Preferences "

Change the >> >> " Default thread sort " to Oldest 

This is a user-to-user forum. You are not speaking to an Apple, or “iStore”, IT team.