_______________main.tf________________

module "m1" {
  source = "../modules/m1"
}

module "m2" {
  source = "../modules/m2"
}
------------------------------------------

------------------m2.tf--------------------
resource "aws_db_subnet_group" "db_subnet_group" {
  name_prefix = "xxx-db"
  subnet_ids  = module.m1.subnetsIds
  tags = {
    "Name" = "xxx db subnet group"
  }
}

--------------------------------------------------


terraform validate

Error: Reference to undeclared module

  on ../modules/m2/m2.tf line 8, in resource "aws_db_subnet_group" "db_subnet_group":
   8:   subnet_ids  = module.m1.subnetsIds

No module call named "m1" is declared in m2.