Hello Karthik,
Upgrade your browser to new version. This is nothing to do with SAP application. its some thing problem with your browser. Meanwhile check if the proxy setting are in place.
Which version of browser you are using, download the latest internet explores (11) and try this.
Also before you press F2, Drag and select the field and then do F2.
Regards,
Nikhil Kulkarni
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