This is a permissions issue.

If it is a Merge push subscription, ensure the Merge Agent process account is db_owner in the distribution database, a member of the PAL, a login associated with a user in the publication database, and has read permissions on the snapshot share.

If it is a Merge pull subscription, ensure the Merge Agent process account is db_owner in the subscription database and that the account used to connect to the Publisher and Distributor is a member of the PAL, a login associated with a user in the publication database, a login associated with a user in the distribution database, and has read permissions on the snapshot share.

All of this is covered in Replication Agent Security Model.

If you are still have problems after this, increase the -HistoryVerboseLevel to get additional details and/or enable verbose agent logging.