If you can’t get Postie to work and you’ve checked all the other FAQs it is likely that you are experiencing a conflict between Postie and another plugin or your theme.
The first thing to try is switching your theme to one of the standard themes such as Twenty Twenty-One. Now trying testing Postie again. If Postie works then you need to get in contact with your theme author to see what the problem is.
If Postie is still not working the next step is to disable all plugins except Postie. Test that Postie works with no other plugins active. If Postie still doesn’t work then something more serious is wrong and you will need to ask for help in the support forum.
Assuming Postie works with all the plugins disabled then next step is to enable one plugin at a time and test to see if Postie is still working. If Postie stops working then you know which plugin caused the conflict and you should contact the plugin author.