Error: Connection was attempted with an older version of SQL Server client communications software that is not compatible with the SQL Server database server
Connecting from ArcGIS Desktop, ArcGIS Pro, or another ArcGIS client to an enterprise geodatabase created within SQL Server returns either of the following error messages:
Connection Error: Connection was attempted with an older version of SQL Server client communications software that is not compatible with the SQL Server database server.
Connection Error: Unable to get database list. A database connection exception has occurred. Unable to connect to database server to retrieve database list; please verify your server name, user name and password info and try again. Connection was attempted with an older version of SQL Server client communications software that is not supported
These errors can occur if SQL Server client drivers are not installed on the PC, or the version of the drivers used are not supported with the version of SQL Server release the user is connecting to. The drivers should be the same or newer than the SQL Server release. For example, Microsoft ODBC Driver 17 for SQL Server is supported with SQL Server 2019, so that should be used when connecting to a SQL Server 2019 instance from ArcGIS.
Though both SQL Server Native Client and ODBC drivers are often backward compatible to older releases, they are not necessarily forward compatible. There isn't any guarantee older/unsupported clients will work without issue. If upgrading the environment, the SQL Server client drivers should be upgraded as well. This is to ensure newer functionality introduced in ArcGIS can be supported.
Solution or Workaround
Verify there is a driver installed on the client PC. If there is, ensure it's the same version or newer than the SQL Server release you are connecting to. ODBC Drivers for SQL Server and the SQL Server Native Client are available for download from Microsoft. Copies can also be found on the My Esri portal under the My Organizations tab > Downloads tab, in the Database Support Files section, as shown below.
Which version of the client files that must be installed depends on the SQL Server release. Refer to the Software required to connect to SQL Server section of the Microsoft SQL Server database requirements for ArcGIS Desktop and ArcGIS Pro page for a full, current list of what is supported.