Not sure what the error could be, you might have used the wrong public key, or you are using a "Non-deterministic watch-only wallet" when you should use a "Deterministic watch-only wallet", as you stated that you're using electrum.
Are you sure you're using your MPK ( Master Public Key) when trying to create a watch-only adress?
https://bitcoinelectrum.com/creating-a-watch-only-wallet/Thank you for responding.
I've done as explained in the link you're referring to. I did it twice (including exporting the MPK) with the same result.