Massaman replacement - unable to find it

They are different but would probably work. Here's a recipe for Massaman curry:
3 cloves minced garlic, 2 teaspoons coriander, 1 teaspoon lemongrass paste, 1 teaspoon salt, ¾ teaspoon black pepper, ½ teaspoon cumin, ½ teaspoon ginger, ⅛ teaspoon nutmeg, ⅛ teaspoon cinnamon, ⅛ teaspoon cardamom, pinch cloves.
You can put it in a blender or food processor to make a paste.
Best,
Karen

Until the recipe authors chime in, maybe this article about Massaman curry (including origins, main tastes, ingredients) will help.
It also recommends (read to end) Mai Ploy Massaman Curry Paste, a reddish brown paste, if you want to make your own.
https://inquiringchef.com/thai-massaman-curry-with-beef/