Each posting schedule has time-slots which are created for specific post types (or any post types). That is why you have to make sure that you have the appropriate post types selected for the time-slots for which you are trying to schedule your posts.

To do that:

Go to Accounts on Publer.
Choose the specific account for which you are scheduling posts right now.
Go to Posting Schedule and check your time-slots.

On the posting schedule above, I have 6 time-slots which are for link, photo, GIF, video, status (text), and any posts respectively.
You can hover over the icon to see what the post type is as well.

If you see that the current time-slot you are using is for link posts only but you are scheduling a photo post, then you won't be able to proceed. Hence, you will get "No available time-slots could be located for this account. Please double-check the posting schedule for this account." as an error message.
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