When I try to use the /give command below in 1.12.2 with a banner, it works fine. The same command generated this error in 18w19b
Also the item minecraft:banner does not appear in the list when typing the command.
In a command block try this.
/give @p minecraft:banner 1 0 {BlockEntityTag:{Base:11,Patterns:[{Pattern:tts,Color:3}]}}
[00:32:13] Unknown item 'minecraft:banner' at position 8: give @p <–[HERE]
Use
minecraft:blue_banner
or whichever is appropriate