If you getting this error - "Error Accessing Store" 

Or in case you have seen in the Store screen a message that alert about a Red cluster status with unassigned shards,

 you should read and do the following steps:

The store is divided into nodes (some used to keep data , others can handle communication and there is one Master that coordinates the work).

To know each node's role you can learn from the Node Type marked in red

Shards need to be allocated/assigned to data nodes so they can be populated with data.

If shards are unassigned to data node no data can flow to the system and this the problem we want to solve.

This situation is usually happens when there is an unexpected recovery of the cluster and at the time of recovery automatic assignment of shards was not applicable for unknown reason.

What can you do ?

There is a script at /app/scripts/realloc_shards.sh <Data Node 1> <Data Node 2> ...

You only need to rake the number at the end of the nodes name for Example es-raw-trans-Node-3 means data node number is 3

The scripts gets a space separated list of data nodes (Those with Type = D) and scan all shards and reallocate unassigned shards to available data node from the list.

In the example above we should run command

/app/scripts/realloc_shards.sh 2

It might take a while but when you refresh the screen you will see the number of unassigned is decrease and the number  of assigned is increased until all are assigned.