I cannot update item because the infopath form open. Is there a way to retreive who is locking the form?

if you just want to retry the action after a set time, I would recommend the 'Update Multiple Items' action, as it offers error handling, in contrast to update item and update field).

Getting the user of the short-term checkoutstatus is more problematic, as it isnt as readily available. You could try the Nintex PowerShell action (although still in beta), and retrieve CheckoutStatus and LockedByUser properties of the file.

Link to the Nintex PowerShell Action post: NTX PowerShell Action - Initial Beta Release