Hello Jomy Theresa Thomas,
Thanks for your question.
Check that both the disk and the VM are located in the same region and, if applicable, the same availability zone
Also confirm it is not an ultra disk as it need specific requirements,
Once done, Can you please try to add the additional disk again by following this Use the portal to attach a data disk to a Linux VM. (if it is aindows one try the equivalent windows steps https://learn.microsoft.com/en-us/azure/virtual-machines/windows/attach-managed-disk-portal)
You can mark it 'Accept Answer' and 'Upvote' if this helped you
Regards,
Abiola