rego_unsafe_var_error: expression is unsafe -- OPA policy

Question

When I run make test on the following rego code:

package policy.ecs.alb_logging

import data.encryption.get_yyy
import data.terraform.library.get_xxx
import input as tfplan

#########
# Policy
#########

xxx_type = "whatever_pqr"

default authz = false

authz {
    created := get_resources_created(xxx_type)
    created == secured
}

secured = num {
    all := get_resources_by_yyy(xxx_type)
    logged := [name |
        name := all[_]
        my_balancer := tfplan[name].attributes
        my_balancer["access_logs.#"] != ""
    ]

    num := count(logged)
}

I see : Error: running verification: build compiler: compiling: 1 error occurred: policy/ecs/whatever_file.rego:22: rego_unsafe_var_error: expression is unsafe

This is for line: logged := [name |

Why is this unsafe? Please help me fix this issue.

I am using conftest 0.20.0