Unable to bulk add users

Hi there,

As I have more than a thousand users registered via other membership plugin, it would not be workable for to assign subscription plan individually.

I’ve clicked the Bulk Add New option but not sure why the whole table is blank. Not one user is shown on the table despite total number of users displayed is correct. There is an option to Edit User when hovering over the blank column Username on each row, but when i click it shows Invalid User ID.

Any idea what could be the reason?
Thanks in advance!