I do not believe an extension was requested on the 2017 return.     The date on the paper copy is 4/3/2018.  If I had to guess it was filed before the April 15th 2018 deadline.   

An amended 2017 return was filed in May of 2020.     IRS states in their letter the amended return was received on May 19th 2020.  

This is the first letter our client has received regarding this.  It is dated May 19, 2021    But looking at the letter, the body of the letter keeps referring to not making an election to forego the 2017 net operating loss, but on the top of the letter it indicates tax period  Dec 31 2018.   So perhaps they are objecting to the 2018 return that was also amended by bringing this passive loss forward from the amended 2017 return.     The 2018 return was also amended back in May of 2020.    

The letter states they will allow 30 days from the date of this letter to file your claims to carry back the 2017 net operating loss.  After 30 days, the three year statute of limitations ( based on the loss year) will expire claims will not be allowed.