Do you refer to the "elgg shop ecommerce" (https://elgg.org/plugins/2723673) and "Argon Theme" https://elgg.org/plugins/3110405 plugins?
They are both for Elgg 2 (2.3) and have never been updated for newer Elgg releases. They won't work on Elgg 3 or Elgg 4. "Fixing" the errors would require to properly update the code of these plugins to work on newer Elgg versions.
It won't help you with the Argon theme plugin but Elgg themes are installed in the same way as other Elgg plugins, i.e. you would copy the theme plugin folder into the mod directory on your server.
Thanks very much!!
I must have downloaded them in the past!!
But thanks for the information. I will be more aware of the version for the plugins, etc.
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