Why do I get "The initial snapshot for publication is not yet available."?
With basic Transactional Replication there are a variety of agents. 

Helpful MSDN article

There is the Snapshot Agent which is responsible for the initial snapshot of data and schema. This can be scheduled to run at regular intervals (I had mine scheduled for every 12 hours).

There is the Log Reader Agent, which reads the transaction logs and sends those which are marked for replication on to the distributor.

Finally, there is the Distribution Agent which is responsible for moving the snapshot(s) and the transactions from the distributor to the subscriber(s).

Your error is telling you that it cannot apply transactions that are marked for replication to the subscriber because the initial snapshot (and/or any of the subsequent snapshots) have not yet been applied. You need to troubleshoot why this snapshot is not being applied. There are many reasons why it could be happening.

Has the Distribution Agent lost permissions to the directory that contains the snapshot files?

Is there some blocking activity on the subscriber that is causing the snapshot to be applied slowly?

Have you overloaded the distributor and/or subscriber by pushing all articles from a large published database to all subscribers at the same time?