Rosetta is a macOS feature that allows programs written for Intel processors to run on Apple Silicon processors.
To use OpenVPN with OpenSSL 1.0.1, which Tunnelblick builds only for Intel and not for Apple Silicon, some versions of the entire Tunnelblick application needed to be run using Rosetta. Setting up the Tunnelblick application to be run using Rosetta was something that the user had to do manually in Finder.
As of Tunnelblick 3.8.8beta03, that is not necessary: when run normally (i.e., not using Rosetta), Tunnelblick will automatically use Rosetta whenever needed, but only when needed. All versions of OpenVPN and OpenSSL/LibreSSL will be available. Thus, Tunnelblick 3.8.8beta03 and higher should not be run using Rosetta.
If you previously turned on "using Rosetta" for the Tunnelblick application, turn it off: