Help in Error: a table with multiple values

Greetings,

I'm trying to create a caluated column but getting the error message (A table of multiple values was supplied where a single value was expected.)

There are 4 tables in the model (listed at the end of the post; tags_information, tags_readings, acknowledgement_reading, & acknowledgement_status).

I'm trying to check to for 2 conditions for each value in the tags_reading table:

1. If the tag meets a certin condition (Running or not running, or another associated value from another table (achknowledgement table) is higher than a specific value).
2. If the tag is lower than a specific limit

I'm creating a calulated column part of the tags_reading table using mulitple variables & LookupValues. The DAX is below.

Trying to troubleshoot the issue, I trying returning each variable to check, I seems the variable (_ack) is returning the error message.

performanceLow_withAcknowledgement = 

var _limit = LOOKUPVALUE(tags_information[Performance Low Limit], tags_information[Tags], tags_readings[Tags])
var _ackTag = LOOKUPVALUE(tags_information[Acknowledgement Tags],tags_information[Tags], tags_readings[Tags])
var _ackType = LOOKUPVALUE(tags_information[Acknowledgement Type],tags_information[Tags], tags_readings[Tags])

var _ackReading =
    IF(
        _ackType = "Value",
        LOOKUPVALUE(
            acknowledgement_reading[Acknowledgement Readings],
            acknowledgement_reading[Acknowledgement Tags],_ackTag),
        IF(
            _ackType = "Status",
            LOOKUPVALUE(
                acknowledgement_status[Acknowledgement Status],
                acknowledgement_status[Acknowledgement Tags],_ackTag)
        )
    )

var _ackLimit = 
    IF(
        _ackType = "Value",
        LOOKUPVALUE(
            tags_information[Acknowledgement Value],
            tags_information[Tags],tags_readings[Tags]),
        IF(
            _ackType = "Status",
            LOOKUPVALUE(
                tags_information[Acknowledgement Value],
                tags_information[Tags],tags_readings[Tags])
        )
    )

var _ack =
    IF(
        _ackType = "Value",
        IF(_ackReading < _ackLimit, "Offline", "Online"),
        IF(
            _ackType = "Status",
            IF(_ackReading = _ackLimit, "Offline", "Online"),
            IF(
                _ackType="Always Online",
                "Online"
            )
        )
    )

RETURN

IF(
    _ack = "Online" && tags_readings[Reading] < _limit,
    "Performance Low"
)

Tables Relationships 

Each Tags_information[Tags] has only one Tags_information[Acknowledgement Tags]
The same [Acknowledgement Tags] might be associated with multiple [Tags]

Tags_information Table

tags_Readings Table

Acknowledgement_Reading Table

Acknowledgement_status table