How to checkout file using PowerShell REST API in SharePoint Server 2013 located in subsite?

I'm trying to check out file which is located inside document library under sub site.

Site URL : mymainsite/subsite/subsite/My Library/test.xlsx

I'm using below PowerShell script

    function CheckOutFile($filePath) {
        Write-Host "CheckOut file is in process"
   
   
        $response = PostRequest ("/_api/web/getFileByServerRelativeUrl('/My Library/test.xlsx')/CheckOut()") (ConvertTo-Json @{ 
            __metadata = @{ type= "SPFile" }
            ServerRelativeUrl = "/My Library/test.xlsx"
        })
    }

**Note:** If I'm trying to checkout in root site library then it is working fine when I'm referring to subsite library it showing "The remote server returend error :(400)Bad request".

What I'm doing wrong here :(