The method you said is only applicable for imported address and not the original address present on the wallet.
I've attached a screenshot below and hope there might be some other way to sign a message from this wallet.
My bad; I tried it on my own wallet before posting, but didn't realize I used an imported address...
The only last option for me would be that I'll share my wallet id with some trusted member over here so that he/she can actually verify what I hold the address.
The only alternative that comes to mind is that if you still have the seed for that wallet you import it on electrum, and from there you signed a message.
If you don't have the seed you can see it again on the security center, as it seems that you are logged in to be able to take that screenshot, and do the above step.