Kelp cannot be placed on most kinds of solid blocks. The only kinds I have found it can be placed on are dirt, coarse dirt, sand, and gravel. If placed on dirt and the dirt changes to grass (due to MCPE-32671), the kelp will break the next time the block is updated.

The wiki doesn't mention that kelp can only be placed on certain blocks. I expect this is an oversight, but we need to know which kinds of blocks kelp is intended to be placeable on.