It has been 21weeks since I filed my amended return. The website says it was completed on August 20 but I still have not received my check.

21 days?

Even during “normal” times it takes about four months for the IRS to process an amended return.  During the pandemic and due to the severe backlog at the IRS it is taking much longer—six months or more for many amended returns.  Do not expect quick results from amending.  When the IRS issues a refund for an amended return it will be by check.  They do not make direct deposits for refunds for amended returns.  You can watch for information here:

https://www.irs.gov/Filing/Individuals/Amended-Returns-(Form-1040-X)/Wheres-My-Amended-Return-1

I mailed amended 2018 and 2019 returns in November 2020 and just got my checks last week (10 months).

Amended returns filed electronically may be processed faster, but there are no guarantees.  It also depends on whether your return was merely accepted into the system, or was actually fully processed with a check approved.  It sounds like the "website" you are referring to must be Turbotax, not the IRS.  Here's how to check amended returns at the IRS web site.

https://www.irs.gov/filing/wheres-my-amended-return

My mistake 21 weeks, not days.