Pomegranate substitute. I don’t have any right now

A lot if people sub cranberries for pomegranates. You could use dried- since fresh aren't in season.

As the pomegranate is only a small amount as garnish, you could just omit it.
But if you want color and taste contrast it would have provided, go for a sweet-sour fruit or syrup, such as
1) dried currants or raisins, rehydrated and with a dash of lemon juice or white wine vinegar.
2) spoon or two of pomegranate molasses.