
Due to some bugs in the initial release of Photos, PowerPhotos can sometimes encounter errors when trying to perform actions on duplicate photos by collecting them into an album or assigning keywords to them. When this occurs, the message displayed by PowerPhotos after it has finished will say that some number of errors occurred. We are working with Apple to try to resolve these issues, but in the meantime, here are some steps you can take to try to get your duplicates in order.
Trying different actions
Behavior in this area seems to vary from library to library, so if you've been using one of the available actions (e.g. "Add to Album") on your photos, try another action instead (e.g. "Assign Keyword"), and you may have more success with that.
Multiple passes
The errors that occur in Photos are often intermittent, so if only some of your actions get successfully applied on your first attempt, making a second pass at marking your duplicates can sometimes help get more of them out of the way. You can also speed things up a bit by using the "Exclusions" settings when setting up your second duplicate search to skip analyzing those photos that have already been marked. For example, if some of your duplicates have had the "duplicate" keyword assigned to them successfully, but others had errors occur, you can check the "Photos that have "duplicate" keyword" in the exclusions panel, and PowerPhotos will skip analyzing them the second time around.
Reduce your library's size
The errors in question tend to happen more frequently the greater the number of photos in your library is, so if there are any photos that you can delete from your library before doing your search, that can help reduce the frequency of errors. There is obviously a bit of a chicken-and-egg problem here, since reducing your library size by removing duplicate photos is what you're trying to achieve in the first place. But, if PowerPhotos can mark some number of your duplicates on one pass, then removing those duplicates before doing your second pass may help.