“CEP” and “extensions” folder does not exist.
Some AE users do not include these folders by default. So you should create the folders manually: Firstly create an empty CEP folder. And then create an empty “extensions” folder inside the CEP folder you have just created. Finally place the “Unreal Backgrounds and Posters Library” folder inside the “extensions” folder.
The Unreal BGP Library extension does not show up in the extensions list in AE.
The extension is either not installed where it should be, or the name of the “Unreal Backgrounds and Posters Library” folder is not “Unreal Backgrounds and Posters Library”. Yes the diagnostic sentence is weird but we could not find a better explanation, sorry for that 
. Make sure that the “Unreal Backgrounds and Posters Library” folder has this exact name and placed under C:\Users\<your_PC_user_name>\AppData\Roaming\Adobe\CEP\extensions\
The “AppData” folder is missing.
Actually it is not. The problem is that it is hidden by default. If this is the case, you should set the hidden folders visibility status to “visible”. The way to do this operation depends on the Windows version you are using. Here https://support.microsoft.com/en-us/windows/view-hidden-files-and-folders-in-windows-10-97fbc472-c603-9d90-91d0-1166d1d9f4b5#:~:text=Open%20File%20Explorer%20from%20the,folders%2C%20and%20drives%20and%20OK. is the instructions for Windows 10 for instance.
When I click the Unreal BGP Library extension in the extensions list in AE, nothing happens, the extension does not start.
The .reg file in the CSXS folder targeting your AE version is not run (you probably run a wrong .reg file). Exit AE, try to run all the .reg files in the CSXS folder (don’t worry, it is completely safe), restart AE and then try running the extension again.
I get the error “AEP file could not be found in bla bla bla …”.
The extension is either not installed where it should be, or the name of the “Unreal Backgrounds and Posters Library” folder is not “Unreal Backgrounds and Posters Library”. Yes the diagnostic sentence is weird but we could not find a better explanation, sorry for that . Make sure that the “Unreal Backgrounds and Posters Library” folder has this exact name and placed under C:\Users\<your_PC_user_name>\AppData\Roaming\Adobe\CEP\extensions\
I can’t find the option “Allow Scripts to Write Files and Access Network”.
Adobe added “Scripting & Expressions” tab to the Preferences menu in AE 2019. For earlier versions, “Allow Scripts to Write Files and Access Network” option is in the “General” tab of the Preferences menu. If your AE version is earlier than 2019, go to Edit->Preferences->General, you will find the required option somewhere in the middle of the list (11. option from above).
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