Steps to reproduce:

• Type /help replaceitem
• This will show /replaceitem <entity|block> ...

Why not specify the'...'

The full syntax should be:
/replaceitem entity <entity> <slot> <item> [count] [data value] [data tag] OR /replaceitem block <x> <y> <z> <slot> <item> [count] [data value] [data tag]