You can launch Invitation, Message, Profile view, Like post, Follow, If accepted, If replied, A/B path, Wait, and End. InMail and Find email can appear in the picker, but Review will block launch until you remove them.
Launchable steps
Add steps on the campaign Sequence tab, then open Review before Launch.
Create or open a campaign and stay on Sequence.
Add Invitation, Message, Profile view, Like post, Follow, Wait, A/B path, or End.
To branch after an Invitation or Message, use the If accepted or If replied fork on that step.
Do not use the Conditions tab. It shows Coming soon. Standalone conditions are not how forks work today.
Open Review. If a row blocks launch, select Fix and change that step or setting.
If accepted and If replied are forks on Invitation and Message. They are not a separate Conditions builder.
InMail and Find email block launch
The picker may show InMail and Find email. They cannot launch.
Review says "InMail sending is not available yet. Remove this step or switch to Message to launch." For Find email it says "Find email is not available yet. Remove this step to launch." Switch InMail to Message, or delete the step.