I hope you are doing well and am sorry for the inconvenience.
– As you can see the error. Enabling wp-json could solve this problem.
– Check for plugin or theme conflicts. Disable other plugins and switch to a default WordPress theme. Then, try to download the file again. If it works, gradually enable plugins and switch back to your theme one by one to identify the conflicting element.
If still the problem persists then share your wp-admin login information here in private reply.