Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, - sql

Question

When I run the following query I get the above error. I realize this is because the subquery is returning more than 1 result but am not sure how to fix it.
update a
set a.covtypeplus = (SELECT distinct
REPLACE(REPLACE(ISNULL([BI],'x') + '+' + ISNULL([PD],'x') + '+' +
ISNULL([COM],'x') + '+' + ISNULL([COMGL],'x') + '+' + ISNULL([COL],'x') + '+' + ISNULL([COLDW],'x') + '+' +
ISNULL([MED],'x') + '+' + ISNULL([PIP],'x') + '+' + ISNULL([PIPNI],'x') + '+' + ISNULL([PIPNR],'x') + '+' +
ISNULL([UM],'x') + '+' + ISNULL([UMBI],'x') + '+' + ISNULL([UMBA],'x') + '+' + ISNULL([UMS],'x') + '+' +
ISNULL([UMPD],'x') + '+' + ISNULL([UMPA],'x') + '+' +
ISNULL([UIM],'x') + '+'+ ISNULL([UIMBI],'x') + '+' + ISNULL([UIMC],'x') + '+' + ISNULL([UIS],'x') + '+' +
ISNULL([UIMPD],'x') + '+' + ISNULL([MEX],'x') + '+' +
ISNULL([ACC],'x') + '+' + ISNULL([CPI],'x') + '+' + ISNULL([INC],'x') + '+' + ISNULL([FUN],'x') + '+' + ISNULL([XMD],'x')
, 'x+', ''), '+x', '') AS CovTypePlus
FROM (SELECT distinct POLICYID, VERSION, COVCODE FROM Staging.Coverage) p LEFT JOIN (
SELECT *
FROM (
SELECT POLICYID, VERSION, COVCODE
FROM Staging.Coverage
WHERE SUBSTRING(COVCODE, 1, 2) IN ('BI','PD','CO','ME','PI','UM','UI','AC','CP','IN','FU','XM','LD','RA','RE','RO','SP','TO','SG','WM')) p
PIVOT (MAX(COVCODE) FOR COVCODE IN (
[BI], [PD], [COM], [COMGL], [COL], [COLDW],
[MED], [PIP], [PIPNI], [PIPNR],
[UM], [UMBI], [UMBA], [UMS], [UMPD], [UMPA],
[UIM], [UIMBI], [UIMC], [UIS], [UIMPD],
[ACC], [CPI], [INC], [FUN], [XMD], [LD], [MEX], [RA], [REN], [ROADS], [SPE], [TOW], [SGC], [WMAR]
)) AS pvt) vc ON p.POLICYID = vc.POLICYID AND p.VERSION = vc.VERSION)
from results_vehicle a
left join staging.coverage c on a.polnum =c.policyid and a.polver = c.version and a.covcode = c.COVCODE
When I run the following original query I created (not an update) it works fine so it must be something I am doing wrong in my update syntax:
SELECT DISTINCT p.POLICYID AS PolNum, p.VERSION As PolVer,
REPLACE(REPLACE(ISNULL([BI],'x') + '+' + ISNULL([PD],'x') + '+' +
ISNULL([COM],'x') + '+' + ISNULL([COMGL],'x') + '+' + ISNULL([COL],'x') + '+' + ISNULL([COLDW],'x') + '+' +
ISNULL([MED],'x') + '+' + ISNULL([PIP],'x') + '+' + ISNULL([PIPNI],'x') + '+' + ISNULL([PIPNR],'x') + '+' +
ISNULL([UM],'x') + '+' + ISNULL([UMBI],'x') + '+' + ISNULL([UMBA],'x') + '+' + ISNULL([UMS],'x') + '+' +
ISNULL([UMPD],'x') + '+' + ISNULL([UMPA],'x') + '+' +
ISNULL([UIM],'x') + '+'+ ISNULL([UIMBI],'x') + '+' + ISNULL([UIMC],'x') + '+' + ISNULL([UIS],'x') + '+' +
ISNULL([UIMPD],'x') + '+' + ISNULL([MEX],'x') + '+' +
ISNULL([ACC],'x') + '+' + ISNULL([CPI],'x') + '+' + ISNULL([INC],'x') + '+' + ISNULL([FUN],'x') + '+' + ISNULL([XMD],'x')
, 'x+', ''), '+x', '') AS CovTypePlus
FROM (SELECT DISTINCT POLICYID, VERSION, COVCODE FROM Staging.Coverage) p LEFT JOIN (
SELECT *
FROM (
SELECT POLICYID, VERSION, COVCODE
FROM Staging.Coverage
WHERE SUBSTRING(COVCODE, 1, 2) IN
('BI','PD','CO','ME','PI','UM','UI','AC','CP','IN','FU','XM','LD','RA','RE','RO','SP','TO','SG','WM')) p
PIVOT (MAX(COVCODE) FOR COVCODE IN (
[BI], [PD], [COM], [COMGL], [COL], [COLDW],
[MED], [PIP], [PIPNI], [PIPNR],
[UM], [UMBI], [UMBA], [UMS], [UMPD], [UMPA],
[UIM], [UIMBI], [UIMC], [UIS], [UIMPD],
[ACC], [CPI], [INC], [FUN], [XMD], [LD], [MEX], [RA], [REN], [ROADS], [SPE], [TOW], [SGC], [WMAR]
)) AS pvt) vc ON p.POLICYID = vc.POLICYID AND p.VERSION = vc.VERSION
Any help is greatly appreciated, I've tried playing around with the query and yield the same results. I also tried adding a SELECT TOP 1 and got the same results for all policies.

Answer 1

You are doing:
update a
set a.covtypeplus = (SELECT distinct . . .
This suggests that you are expecting more than one result from that subquery. Not allowed.
Perhaps this will fix it, but it may not be what you want:
update a
set a.covtypeplus = (SELECT top 1 . . .

Answer 2

It appears to be this part that is messing you up:
from results_vehicle a
left join staging.coverage c on a.polnum =c.policyid
and a.polver = c.version and a.covcode = c.COVCODE
This must be generating more than one result for some values of a.covcode.