Error while programmatically creating a site from web part SPO

I'm trying programmatically to create a site from a web part, sandbox solution for SharePoint Online 2010. My web part has a text box which displays error messages and a button Create, in which on click event creates a site.

If the parameters of my method are

web.Webs.Add("TestAccount", "TestAccount", String.Empty, web.Language, SPWebTemplate.WebTemplateSTS, false, **true**);

Whether the template is my custom template or SharePoint site template (Team Site), I get an error:

"Cannot convert a folder if a web template is specified."

If the parameters of my method are

web.Webs.Add("TestAccount", "TestAccount", String.Empty, web.Language, SPWebTemplate.WebTemplateSTS, false, **false**);

I get an error:

"The web site address is already in use."

Even though there is no site in the workspaces under TestAccount name and it has not been created.

using (SPSite site = SPContext.Current.Site)
        {
            using (SPWeb web = SPContext.Current.Web)
            {
                webTemplates = site.RootWeb.GetAvailableWebTemplates(1033);
                foreach (SPWebTemplate webTemplate in webTemplates)
                {
                    if (webTemplate.Title == "Engagement Site")
                    {
                        myTemplate = webTemplate;
                    }
                }
                 bool allowUnsafeUpdates = web.AllowUnsafeUpdates;
                    web.AllowUnsafeUpdates = true;
                    accountSite = web.Webs.Add("TestAccount", "TestAccount", String.Empty, web.Language, SPWebTemplate.WebTemplateSTS, false, true);

                    accountSite.Update();
                    web.AllowUnsafeUpdates = allowUnsafeUpdates;
            }
        }