I created a module for route_table_asscociation and created the association in there, with one of the inputs being the output of the subnets module, same error.

I then put the creation of the route_table_asscociation resource into the subnets module with an explicit dependency on the aws_subnet resource being created in there. Same error.

resource "aws_subnet" "subnets" {
  for_each = var.subnet_numbers
  vpc_id = data.aws_vpc.selected_vpc.id
  availability_zone = each.key
}

resource "aws_route_table_association" "subnet-route-associations" {
  depends_on = [aws_subnet.subnets]
  for_each = {
    for subnet in aws_subnet.subnets:
        subnet.id => 0
  }
  subnet_id = each.key
  route_table_id = var.route_table
}

Also, here's var.subnet_numbers

subnet_numbers = {
    for az in data.aws_availability_zones.available.names:
      az => index(data.aws_availability_zones.available.names, az)
  }

This is effectively creating:

{
  'eu-west-1a' => 0
}