Why can't I download apps on my client's website?

Hi there,

I am working on a client website and the theme of the website has not been updated for a while. I tried to do this manually but when I replace the code in settings_data.json, it is showing a JSON error. So option 2 was to install an app to update the theme to its current version, however I cannot add any apps. I have contacted the clients and she gave me the correct permissions and it is still not working. Then she tried to add the app which did not work either. I have done all the usual things such as deleting cache, history, incognito window. Plus I have tried to download apps in the account of my other clients (in the same Chrome window) and this works fine. Makes me think it is a problem with my clients account. Would it be because the theme have not been updated or is it just a simple setting that I am missing.

Below the error message I see when I try to download an app.

Has anyone experienced this before or know how to solve this?

Cheers,

Marije