Filing Self Assessment if the year in question has not been issued by HMRC

Please can you advise if we can file self assessments if the year in question is not issued or do we need to contact HMRC to open the year before we file? Thank you

Hi Robert 
You can send a return for a year that has not been issued, but any such return received will take longer to process.
Requesting to file for a year reduces the time taken to process your return and make any repayments due available -
to do so, contact us by webchat or phone via Self Assessment: general enquiries
Thank you.