MS SQL Error 26 in MS SQL Express 2016
SQL Server Named instances by default are running on a dynamic TCP/IP port (so it won't be default 1433). You can try to do a few things to enable access to your instance (assuming that Firewall is configured properly). 

SQL Server Browser

You need to ensure that the SQL Server Browser service is running. 


Open SQL Server Configuration Manager
Go to SQL Server Services
Right-click on SQL Server Browser service
Click Start




Try to connect to your instance providing &lt;hostname&gt;\&lt;instancename&gt; or &lt;IP&gt;\&lt;instancename&gt; as Server Name. If that doesn't help go to the next step.

TCP\IP Protocol

To enable TCP\IP protocol for your instance:


Open SQL Server Configuration Manager
Go to SQL Server Network Configuration
Go to Protocols for 
Right-click on TCP/IP
Click Enable
Restart your SQL Server instance




If you want to use a static port for your instance (instead of dynamic that changes after every restart) you can change it here.


Open Properties for TCP/IP protocol
Go to IP Addresses tab
Scroll down to IPAll section
Remove 0 value from TCP Dynamic Ports
Specify your port in TCP Port




You can use this port to connect to your instance by providing &lt;servername&gt;,&lt;port&gt; or &lt;IP&gt;,&lt;port&gt; as Server Name (yes, there is a comma, not a colon).