Which version of Episerver you are using?
Please see the answer of below thread if it helps-
"For 7.5, it's required that Apps folder (which path is defined in CommerceManagerSharedPath) to be a virtual directory under Commerce Manager IIS site. Do you check in IIS Manager to see if it's there?"
In order to run the commerce site and access commerce manager you need to have two separate sites sites running - one for Episerver and 1 for commerce manager. The web.config for the CMS site has a setting which tells it which URL to use to access the separate Commerce Manager site. Are you able to access the commerce manager site directly via its own URL rather than via the URL you posted (as that's the URL for the CMS UI wrapper around commerce manager)? If not, it sounds like you need to set up commerce manager in IIS too under its own URL.
If you can access commerce manager directly but can't access it via the CMS UI, it could be that the URL for commerce manager isn't configured correctly in the CMS site's web.config.
As a rule of thumb your CommerceManager version and your Commerce.Core version should match. It is not a hard requirement, but it will be easier that way.
Also second what Paul said, you should check CommerceManagerLink appSettings in web.config to make sure it is point to the site hosted in IIS, as you are access it via the CommerceManager Integration plugin. You should be able to access the site directly.
@Paul Gruffydd @Quan Mai Does that mean we must have 2 separate Url's for Commerce manager and CMS?
I have now hosted Commerce manager separately and changed Url in the web.config of both site and Commerce manager. But still I am not able to open it and getting error "localhost refused to connect". However, I can open by entering Url directly. Did I miss anything else?
Yes, they are two separate sites. When you are accessing via http://sample.com/EPiServer/EPiServer.Commerce.Shell.ManagerIntegration/Admin/SitePlugin/CommerceManager.aspx it is loading commerce manager site using iframe. So you should have sample.com for cms and samplecm.com for commerce manager
Commerce manager page is not loading when hosted in IIS; url is as shown below. However it loads in localhost. There is no log in App_data folder.
I am using EpiServer.Commerce.Core of version 13.3.1 and EpiServer.CommerceManager of version 13.2.0.