SQL recovery mode Change to full

Before adding any exiting databases to always on group, it is required to change the recovery type to Full and perform a full backup.

If the recovery type is set Simple then when adding the database to Always on Group, the options to add will be grayed out and the status will display Full recovery mode is required.

To change the backup type and do a full backup;

Connect to the SQL server instance using SQL management studio then expand the databases node in the object explorer then select ConfigMgr database then right click and select properties;

On the database properties go to options;
By default the recovery model is set to simple;

Change the recovery model to Full then click OK

Now, right click on the ConfigMgr database then select Tasks then Back Up;
It will open Back Up database window;

It is not required to change anything on Back Up Database window, click OK,
This will start backing up the database;

Wait until ‘The backup of database completed successfully’ message;

That's It.

Now we are ready to add this database to SQL AlwaysOn Available Group.

Also refer to the complete guide for SCCM SQL AlwaysOn Availability Group guide.