Regarding CP2000 Notice I received from IRS last year for year 2018. I forgot to sign and mail it by the deadline, IRS owes me an additional refund, can I still claim it

Not sure how a CP2000 for missing income can get you an increase in your refund ... it usually means you OWE the IRS and need to pay them ...  so read that notice again carefully.   

If  it really  did say you are due a 2018  refund  then although   I doubt it  will be paid out at this late date  you could try signing it  and mailing it  in as directed ... can't  hurt to try. 

If you want your original refund, and IRS is waiting for your signed return, you probably will get that.

If you want to amend for additional refund, it is past the deadline for that unless you had filed for a 2018 extension.

@sunitapatial