[SalesForce] Emails not being sent

The first thing that I would try is to create a Sendable Data Extension containing the fields and values that you are using in your API request.

Validate the email using the Sendable DE you created. If there are no errors, then use the Send Preview feature and preview the records in your DE — my guess is you're using AMPscript in your email and there's a data related issue where values are failing to render by the OMM process when the email is sent.

Emails configured in Journey Builder are Triggered Sends, which you can find in the Interactions tab in SFMC.

You can pause the trigger corresponding to your email and download the queued subscribers or clear the queue. You can also pause and restart the trigger definition in case one email is holding up the queue.

I'm guessing there's some issue with the data coming over in the payload that isn't handled in the email scripting.

Edit

I'd revise your AMPScript to make it more fault-tolerant and to include the CustomerID which is required in the DE you're updating:

%%[

var @eid, @js, @cid

set @eid = AttributeValue("EventInstanceId")
set @cid = AttributeValue("CustomerId")

if empty(@cid) then
  set @cid = "0"
endif

set @js = now()

UpsertDE("Account Updates", 1, "EventInstanceId", @eid , "CustomerID", @cid, "Journey Status", @js) 

]%%

It's a good practice to code your email assuming that any of the incoming values can be null. The AttributeValue() function will return an empty string if the personalization string is undefined.