I thought I was supposed to receive income tax return 21 days later I mailed it February 8 and still have not received it in the mail

No keep waiting.  21 Days is just an estimate.  Mailed returns are taking much longer this year.  Like 6 months or more.  They just started processing mailed returns.  

@amazinglyclean00 DO NOT REFILE!!! that will just make things worse. 

the IRS processes everything in the order it is received.  They just announced on Tuesday that they had completed all paper tax returns received prior to 1/1/22....so that means they have NOT even begun processing 2021 tax returns that were submitted in paper form!!!! 

as noted by @VolvoGirl , assume it's going to be 6-9 months from February before you see anything. 

IRS continues work on inventory of tax returns; original tax returns filed in 2021 to be completed this week | Internal Revenue Service

Hold on and wait.

I mailed my 2020 return in October 2021, priority mail.

Never got record of delivery, but just last week, IRS.gov site, I found that the return had been processed.

21 days is an estimated for efiled returns.   It does not apply to mailed ones.