I want create various VLAN in OpenWRT 18.06.1 and I have readed that the first think that I need to do is activate the VLAN support in the "Network -> Switch" option menu but it not appear in my options menu.
If you have CPU ports that go directly to outside, as most x86 boxes do, you create VLANs directly on the port with notation like 'eth0.3' instead of 'eth0'.
Also some high-end chips use a "DSA" type of switch driver, which abstracts the hardware to look like there are separate eth ports that go directly outside. To the user's perspective, think of it as there being no switch.
The AR9331 chip has an integral 5 port switch in between its CPU port and the Ethernet hardware ports. Router models built with fewer than 5 ports simply leave some of the pins of the AR9331 unconnected. Even with only one connected port, the switch still has to be configured in order to pass traffic through.
If the swconfig command exists on your router, the firstboot scripts should have built out a basic config in /etc/config/network on the first boot. Try using swconfig dev switch0 show to see if / how the switch is configured.