When I try to replace air blocks in an area with beacon blocks with this command:
"/fill ~1 ~ ~ ~-1 ~-1 ~ minecraft:beacon 0 replace minecraft:air"
I get this error: "Data tag parsing failed: Invalid tag encountered, expected '{' as first char."

The command works with other materials:
"/fill ~1 ~ ~ ~-1 ~-1 ~ minecraft:glass 0 replace minecraft:air"
for exemple.

Replacing minecraft:beacon with a different material works and placing beacon blocks without replacing any material works too.