Flows in Klaviyo do not have an on/off switch. Each individual email in a flow series has it's own send status: Draft, Manual, or Live. This allows you to build out a flow series and enable certain emails to start sending while still finalizing other emails you may want to enable later on.
If you would like to enable an entire flow to start sending, you will need to turn each email Live one-by-one.
Turn a Flow Email Live
For each email in the flow you want to start sending, make sure you've customized and previewed the content and all settings. Next, in the lefthand configuration sidebar, click the Send Status dropdown and change the status to Live. You will notice the icon on the email card update to turn green as well.
Turn an email Live means new individuals entering your flow will be queued up to receive this email and it will send out automatically at the right time. Updating the send status of an email won't affect anyone who might have received this email in the past.
If this email's status was Manual and there are any recipients still awaiting review, those recipients will still need to be reviewed and sent manually. Emails marked for manual review are not sent automatically when an email's status changes to Live.