When exactly are you getting that? I'd guess it's google bot protection (though that shouldn't ideally be a 403, but who knows what they're doing), but I'm surprised you'd see that screen?
i am not able to import citation from google scholar. It shows error 403. Initially I was directly able to download citation automatically in one click and add in MS WORD. But now it shows this error and I have to manually work on everything.
Do you mean that you're clicking the Cite button on Google Scholar? Same answer — it's just Google rate-limiting you — but note that that's not how you would add things to Zotero on a supported site like Google Scholar. You just click the save button in your browser toolbar.
Thank you so much
https://www.zotero.org/support/adding_items_to_zotero