You need to be a site collection administrator to set this property.

Issue Description

Recently I was trying to add a site collection administrator to a site and I got the belowmessage

You need to be a site collection administrator to set this property.   at Microsoft.SharePoint.ApplicationPages.ManageSiteAdminPage.BtnSubmit_Click(Object sender, EventArgs e)

   at System.Web.UI.WebControls.Button.OnClick(EventArgs e)

   at System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument)

   at System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument)

   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

I was already the site collection administrator to the site and it did not make any sense.

Fix

On further trouble shooting, I found the solution for this.Browse to Central Administration - > Site Collection Quotas and Locks.

The options available here are:

       Not locked

Adding content prevented

Read-only (blocks additions, updates, and deletions)

No access

This site collection was in Read-only status. Unlock it (Change the status to  "Not Locked")