RecordType

Either do what Stefen  said or pass in value like this { fv!item.id}.

Tip: I generally create query with operator as IN always, so that in future we don't have to worry if the passed parameter changed to multiple 

I tried with Operator "in" and "=", still getting same error.

Please find the below lines of code which I used inside RecordType userfilter configuration.

a!localVariables(
local!attorneys: rule!LCM_getAttorneys(soeid: null, attorneykey: null()),
a!recordFilterList(
name: "Attorneys",
options: a!forEach(
items: local!attorneys,
expression: a!recordFilterListOption(
id: fv!item.attorneykey,
name: fv!item.name,
filter: a!queryFilter(
Field: 'recordType!{53d820bb-ea8a-4eb8-be61-11f5d777ee73}LCM.relationships.{7284648d-721f-4542-b1be-33ec9de327a9}recordattorneys.relationships.{fa2cff10-cacd-45cf-80a9-9c281abc512c}attorneys.fields.{6b8c7aae-1e6d-4313-9ced-93ccaed0d5da}attorneykey',
operator: "=",
value: {fv!item.id}
)
)
)
)
)

I tried with Operator "in" and "=", still getting same error.

Please find the below lines of code which I used inside RecordType userfilter configuration.

a!localVariables(
local!attorneys: rule!LCM_getAttorneys(soeid: null, attorneykey: null()),
a!recordFilterList(
name: "Attorneys",
options: a!forEach(
items: local!attorneys,
expression: a!recordFilterListOption(
id: fv!item.attorneykey,
name: fv!item.name,
filter: a!queryFilter(
Field: 'recordType!{53d820bb-ea8a-4eb8-be61-11f5d777ee73}LCM.relationships.{7284648d-721f-4542-b1be-33ec9de327a9}recordattorneys.relationships.{fa2cff10-cacd-45cf-80a9-9c281abc512c}attorneys.fields.{6b8c7aae-1e6d-4313-9ced-93ccaed0d5da}attorneykey',
operator: "=",
value: {fv!item.id}
)
)
)
)
)

here fv!item.id means local!attorneys.id 

So is that multiple type?

This is user filter dropdown with multi selection. 

Why do you wrap fv!item.id into curly brackets? This makes it a list. Comparing that using the "=" operator will fail.

Either use "in" and a list as value, or "=" and a single value. And make sure that the data types match.

If multiple then make it in operator. its simple logic

if value to be passed is single then use "=" else use  "in"