You need to be sure to use the update of the watchface associated with wear os 6.
You can check this on the play store of both watch and phone, by searching by the watchface name.
Once it is installed on both devices, use the VERIFY button: it should display a screen on your watch to check the installation status. If validated, then you can use the watchface on your watch, by selecting it in the watchface list.
On your watch, while on the main time screen, hold your finger on the screen, and it will display the watchface list. Search for the watchface in it.
If not available, go to the last right item (« +add watchface »), select it, then scroll until you find the « downloaded » section. The watchface should be available here. Just select it.