Databases that will be part of an Availability Group need to have Full Recovery Model, and Backed up.
Within SQL Server Management Studio (SSMS), expand Databases -> right-click on SampleDB and click on Properties:
In the Database Properties area, go to Options then make sure to change the Recovery Model to Full (You can also change the Compatibility Level to the latest - 2019), the hit OK for the changes to take effect:
Open up a new query window and execute the below script to initiate a full backup:
BACKUP DATABASE [SampleDB] TO DISK=N'Z:\MSSQL15.MSSQLSERVER\MSSQL\Backup\SampleDB_new.bak'
SampleDB is now ready to be added to an AlwaysON Availability Group