It is seriously frustrating to install a 3rd party extension into Mijoshop, only to get the error “The page you are looking for could not be found! Please contact your administrator if the problem persists.” afterwards. There are generally two ways to fix this:
1. Access Issue
Some extensions require that you give permissions first. Go to users->user groups->Administrators and then for both Access Permissions and Modify Perrmisions, hit Unselect All, then Select All.
2. Corrupted modules folder
In FTP, go to /public_html/components/com_mijoshop/opencart/admin/controller/extension and remove /modules/ folder (it will re-generate).