Support for Exchange 2019 starts with NBU 8.2. Have a look at the compatibility list here: https://www.veritas.com/support/en_US/article.000033647
You will need to upgrade as Veritas will not add support for version 7.7.x.
Exchange GRT will not work with NetBackup 7.7.3 and Exchange 2016 or 2019.
Even without GRT, Exchange Discovery needs a workaround for Windows 2016 or later, until NetBackup 8.2.