Red Nether Brick Slab is a decorative half-block crafted from Red Nether Bricks, which are themselves made by combining Nether Wart and Nether Bricks. These slabs can be placed horizontally against the top or bottom of other blocks, effectively creating a half-height space that players can use for a variety of building purposes, such as creating shallower steps, more nuanced architectural features, or simply as a stylistic building element.
This item has no numerical item ID, therefore you must use the item ID to spawn the item. Numerical item IDs were removed in Minecraft version 1.13.
The item id for Red Nether Brick Slab is minecraft:red_nether_brick_slab
The item ID for Red Nether Brick Slab is minecraft:red_nether_brick_slab
This is the spawn command to give yourself Red Nether Brick Slab in Minecraft which includes the Minecraft /give command. If you are using the Essentials plugin you will need to use /minecraft:give instead of /give