To understand the issue you are having
Video: Syncing in Anki: One-Way Syncs - YouTube
Although it should not happen often, occasionally you may end up in the position where your cards on AnkiDroid can not be automatically merged with the cards on AnkiWeb. In this case it’s necessary to choose to either upload to or download from AnkiWeb, which would overwrite any changes on the other side.
Most of the issues happens when changing the note types (importing, creating fields…)
There are certain changes that Anki is unable to merge. These mainly relate to the format of notes: things like adding a new field, or removing a card template. When you perform an operation that can’t be merged, Anki will warn you, and give you the option of aborting the operation. If you choose to continue, you’ll be asked to choose whether to keep the local copy or the copy on AnkiWeb when your collection is next synchronized.
Could happen when downloading new decks as well
Note that if you have any existing material in AnkiDroid before attempting to sync, you may be shown a message asking you to choose to either download from AnkiWeb, or upload to AnkiWeb. If you are happy to lose the cards in AnkiDroid then simply choose Download. If you need to merge the existing cards with AnkiDroid then you should see the resolving conflicts section before continuing.
Most of the warnings is on Anki side, since Ankidroid is mostly used for reviews
the sync will display in orange to show users that a one way sync will happen
if you change the notetype, like mentioned in the video above, this warning will pop up
The requested change will require a full upload of the database when you next synchronize your collection. If you have reviews or other changes waiting on another device that haven’t been
synchronized here yet, they will be lost.
If you have chosen the wrong option to sync, you could use the backup on anki side
Lets say you want to merge both sync because you made changes to both of them, then Merging Conflicts