Hi @Silja M C,
According to your description you mean that you cannot type anything in the "Enter the Product Key" textbox, right?
Please try run the SharePoint Central Administration as administrator. Also, try change a browser or access the SharePoint Central Administration on another server.
If you bought SharePoint directly from Microsoft, you can go to SharePoint volume licensing service center to check the expiration date. If you bought form MS partner, you need to contact them to help you check.
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