ACE is not included in the MSFS Marketplace version. You can download the free liveries directly from various flight sim portals.

If you have CS store version:
Run ACE (not included in the MSFS Market Place version) Start > Captain Sim > 777 ACE MSFS2020

To add a new livery:
(o) Online > download > Save

To delete a livery:
(o) Local > select one > Delete

If you can't find/run ACE