I have already contacted with hosting company Hostgator. Since it is a shared hosting they will not provide any root command to be executed. I have got a huge warning like:
[PHP] Error: Version warning: Imagick was compiled against Image Magick version 1650 but version 1654 is loaded. Imagick will run but may behave surprisingly at Unknown#0
GET /nextcloud/ocs/v2.php/apps/notifications/api/v2/notifications
from 27.147.202.170 by admin at 2020-04-06T19:21:31+00:00
How to get rid of this.
Thanks 
Ask your hoster to fix the problem and update the php module of your web server so that it matches the underlying Image Magick version.
As I said, i am using shared host. They said the message is occurring due to the custom configuration of the shared server. I was able to ignore and surpress the message by adding imagick.skip_version_check=1 this to .user.ini.
I think for solving this, I need to switch dedicated host. 
You dont need to a dedicated host. The problem is that the update Image Magick but forgot to compile php with the new version. Ask them to update the php versions again, and the problem is gone.
Indeed, I see some of my troubleshoots failed due to this as well. Very difficult to work with a shared host.
The recommended minimum memory limit to use is 512 MB, otherwise messages similar to the displayed one might appear.
