GST registration

Check POP-UP-BLOCKER, it might be ON.

Allow the pop-up for the web.

The GST Portal has not yet been linked to UIDAI i.e. Adhar Signature. There is some issue with this functionality. Also, there is no cut-off date when this option of sign shall become available. 

Even on perusal of helpline, they would tell you to submit it without any sign and keep trying everyday if e-sign option is activated and submit with e-sign later on.

In my opinion, approach the client and DSC Provider. I am not saying obatin digital signature but be prepared. In case even if till the last date, e-sign is not activated, you must have a backup. I mean collect documents for DSC and also consult DSC Provider as how many days minimum will he need. So that we can inform him accrodingly.