I imagine your definition of Render looks like this
In .cpp
void Texture2D::Render(Vector2D newPosition, SDL_RendererFlip flip, double angle = 0.0f)
{
...
}
When it should be
void Texture2D::Render(Vector2D newPosition, SDL_RendererFlip flip, double angle )
{
...
}
You only supply the default value in the declaration ( normally .h )