I get this kind of error message after adding the first person mod for the witcher. Could someone tell how to solve this?

Error [modactivewitcherzoom]game\vehicles\boat\boat.ws(119): Native function 'HasDrowned' cannot have script code.
Error [modactivewitcherzoom]game\vehicles\boat\boat.ws(120): Native function 'SetHasDrowned' cannot have script code.

Warning [content0]engine\environment.ws(25): Global native function 'EnableDebugOverlayFilter' was not exported from C++ code.
Warning [content0]engine\environment.ws(27): Global native function 'EnableDebugPostProcess' was not exported from C++ code.
Warning [content0]engine\showflags.ws(9): Global native function 'DebugSetEShowFlag' was not exported from C++ code.

Which mod do you intend to use, and is the mod compatible with your game version? If for example the game version is 1.21 (newest), then the mod needs to be updated to game version 1.21 as well or code conflicts will be the result.

From the description of that mod you're using:

"Active Witcher Zoom uses its own script file, but it also modifies the following core script files:
scripts\game\player\r4Player.ws
scripts\game\player\states\swimming.ws
scripts\game\player\states\vehicles\dismountTheVehicle.ws
scripts\game\player\states\vehicles\horseRiding.ws
scripts\game\player\states\vehicles\mountBoat.ws
scripts\game\player\states\vehicles\mountTheVehicle.ws
scripts\game\player\states\vehicles\sailing.ws
scripts\game\vehicles\boat\boat.ws"

You must use script merger.

TC did not mention any other already installed mods, so...

The script merger is only needed if several mods use the same file, so a single mod wouldn't throw the towel like that.

The description page and file page of the mod on nexus don't specify which game version the mod is updated for. Seeing that the last update was back in September I highly doubt that the mod is compatible to game version 1.21...

Yes it does:
"latest mod version. Updated for compatibility with v1.21"