The SQL Server Native Client is missing from the Sage Estimating 21.1 Prerequisites. You will see something like this in the preppy log file:

08:18:31.632 [package # 2][X64] Installing SQL Server Native Client (x64): msiexec /i "C:\Installs\SE 21.1 (2021_09_15)\Prerequisites\SQL Server\2012.2\x64\sqlncli.msi" /qn /l*v

If you look at the Sage Estimating 21.1 installation file folder, you will see that .\2012.2\x64\sqlncli.msi does not exist:

The SQL Server Native Client has to be downloaded separately and deployed before Sage Estimating will successfully run.

Note that the is also available from earlier CD images, as shown below in the SE 20.1 install folder:

Also see article Open Estimate Failed