SQL Server Edition and Always On Setup

Standard Edition only allows Always On setup in basic mode. Switch to Enterprise Edition for full Always On features.

Always On Basic Mode (Standard Edition) Limitations

Basic availability groups use a subset of features compared to advanced availability groups on SQL Server Enterprise Edition. Basic availability groups include the following limitations:

• Limit of two replicas (primary and secondary). Basic Availability Groups for SQL Server 2017 on Linux support an additional configuration only replica.
• No read access on secondary replica.
• No backups on secondary replica.
• No integrity checks on secondary replicas.
• No support for replicas hosted on servers running a version of SQL Server prior to SQL Server 2016 Community Technology Preview 3 (CTP3).
• Support for one availability database. For multiple databases, you have to setup multiple AG groups.
• Basic availability groups cannot be upgraded to advanced availability groups. The group must be dropped and re-added to a group that contains servers running only SQL Server 2016 Enterprise Edition.
• Basic availability groups are only supported for Standard Edition servers.
• Basic availability groups cannot be part of a distributed availability group.
• You may have multiple Basic availability groups connected to a single instance of SQL Server.

Sources

https://learn.microsoft.com/en-us/sql/database-engine/availability-groups/windows/basic-availability-groups-always-on-availability-groups?view=sql-server-ver16