GST and ITC availment

ITC can be availed on the basis of self assessment.

However , where supplier fails to put invoices in GSTR_1 or  entered the invoices but  failed to file the return so input can be availed upti 20% of total eligible input 

my question is ITC availment is optional or not.

I can show full value of invoice  as expenses and not availing ITC. 

Yes you can book full value of invoice as expense and not avail ITC. Availing ITC is your Right as per LAW and its depend on you whether you want to avail or not. 

In the light of explained factual position, it is suggested you not avail ITC of such invoice