I have MSFS2020 installed in E:\Flight Sims\Microsoft Flight Simulator, and the CS 777 installed into the E:\Flight Sims\Microsoft Flight Simulator\Community folder, so the chances are that the installation folder the CS777 for you is D:\MSFS2020\Community folder. So if you navigate to the D:\MSFS2020 folder using Windows Explorer, you can see if the Cominunity folder is in there. If It is, open it and check to see if the captainsim-aircraft-m772 is in there. If it is:
On the ACE, click on the button beside the Select Official Folder, with the ... in. Then navigate to the D:\MSFS2020\Community folder, select the captainsim-aircraft-m772 folder, click OK (or whatever it is that shows up so tht the folder is selected). When back in the ACE, with the location showing in the ACE, click on the Save button to save the location. Hopefully after that, the ACE will work, and you can then add liveries and such.