VK: Access denied: could not access to this page

If you see this error when trying to reply from the service, it is likely that the user who connected the page to the service is no longer the admin of this page and cannot reply on its behalf.
In this case, you need to reauthorize in your Social profile settings by the VK user who is currently the admin of your page.
To do this, go toSettings - Social profiles, select desired profile and click on Authorize the Group/Community.

Authorize a group