Cannot transfer a great admiral to another city

Solution 1:

As obvious as it is or is not. When the admiral spawns, it was in this case, it was a harbour district. (this was during my first game, so I am now well aware of this shift of units preferring districts when producing)

Once I moved the admiral to a city, I then had the option enabled to transfer to another city.

Much the same as if building a naval unit that if a coastal city has a harbour district, the unit is built there.

Note, you need a city on the coast to be able to move the great admiral.

Solution 2:

Here is a workaround allowing to move your great admiral as if it was a land unit:

1. Open the file "Units.xml" (it is located in ..\Base\Assets\Gameplay\Data\Units.xml)
2. Make a copy of Units.xml if you like to play it safe
3. Go to the line with UnitType="UNIT_GREAT_ADMIRAL" in the section "Units"
4. Change "DOMAIN_SEA" to "DOMAIN_LAND"
5. Start or restart the Civilization VI and load your save game
6. Be aware that you won't see the change until the next turn

Source: https://forums.civfanatics.com/threads/bug-great-admiral-spawned-in-a-lake-and-cannot-leave.603207/#post-14850070