There are 5 steps of email creation in Prowly and the Send step is the final one. If you cannot proceed from the 4th step - Review - with the Next buttons continuously greyed out, it clearly means there are critical errors in your email that have to be fixed before going to distribution.
Clicking the Resolve button next to a specific error will immediately allow you to fix the problem or will take you to the place in the email creator where it can be fixed:
When Errors are moved to the Done section, they will receive green indicators, which means you're ready to move on.
At this stage, you can also find valuable suggestions that you may - but don't have to - use for email optimization.