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Hello,

Pls let me know which extension you did install ?

I’m using this extension.
https://www.mgt-commerce.com/magento-delete-orders.html

You can try to remove/clear extension in app folder. Then login to admin site again > re-install extension:

1. Disable Cache in admin backend System -> Cache Management
2. Logout Admin panel.
3. Up code extension to folder app
4. Login Admin

I tried to install extension and it worked fine.

Excuse me, how to remove/clear extension in app folder? Which exact folder for this extension?

You access to ftp and delete files of extension:

app\etc\modules\Mgt_Base.xml
app\etc\modules\Mgt_DeleteOrders.xml
app\code\community\Mgt
app\code\local\Mgt

Then log into Backend > Disable cache in System > Cache Management > Logout

and upload files of extension again to re-install it.