Pervasive 8.6 is really really old. Some 15 years old I would guess. At that time there was no 64bit version for PSQL.

The current latest version is v15. Actian only supports v13 and later

For DataFlex 20.0 64bit programs you need a 64 bit PSQL Client
For DataFlex 20.0 32bit programs you need a 32 bit PSQL Client
Since the DataFlex 20 Studio is a 64bit application, you need a 64bit PSQL client to run the DataFlex 20 Studio.

All the latest PSQL (Or Actian Zen as it is called nowadays) client versions will install both the 32bit and 64bit drivers.
-32bit and 64bit ODBC drivers
-32bit and 64bit Btrieve divers

Note the DataFlex PSQL driver (DFBTRDRV) does not use ODBC, but the Btrieve API to access PSQL database.
The Btrieve Client drivers are (by default) located as follows:
32bit C:\Program Files (x86)\Actian\PSQL\bin\w3btrv7.dll
64bit C:\Program Files\Actian\PSQL\bin\w64btrv.dll

These folders need to be in your PATH environment variable, for the DataFlex PSQL driver (DFBTRDRV) to find them.
If these dll's cannot be found, you will get the "Can't load DLL DFBTRDRV" error.