You'll need to make sure that the group you want to post in has the Sonlet app installed before posting to Facebook. This is a new requirement from Facebook as of the beginning of August 2018. You will need to install the app from a computer, from Facebook through your browser.
Installing or reinstalling the app is the most common solution to the error: 'post' error:  (#200) Insufficient permission to create album
A video demo for the process is available here: https://www.useloom.com/share/2125f39719164592bd3f66fbd48a2f17
Here's how to do it:
First, go to your group's edit page by clicking "...More" on the group page and selecting "Edit Group Settings".
Once you're on the group edit page, scroll down to find the app section and click "add apps".
A window will pop up and you can use the search field to find the Sonlet app; once it appears in the filtered search results, go ahead and click it.
Another window will pop up allowing you to select Sonlet for your group; click "add":
Now Sonlet should display in your apps:
(You can view Facebook's instructions for installing an app in your group here.)
After installing the app in your Facebook group(s), we recommend logging out of Facebook by clicking the blue Log Out button on your Facebook Doctor page and logging in again to grant permissions and establish a fresh connection. Detailed instructions are available in this article.
This process should make it possible to post to Facebook. Please don't hesitate to reach out if you have any questions along the way.