SQL Server: Dynamically Pivot Temp Table (Dynamic SQL) - sql-server

Question

I want to Dynamically pivot my #Tempproj table. This table consists of multiple CTE's that I insert into this #Tempproj. Now I want to Pivot this temptable, however I currently can only do it now when I create a Physical table instead of a Temporary one.
When I run it with the following query it says:
Must declare the table variable "#Tempproj"
I don't know how to mix this SQL with Dynamic SQL correctly.
First I declare a temp table, quite a long statement, and all the way below is the Dynamic SQL Pivot query I use.
CREATE TABLE #Tempproj
(
[ProjKey] nvarchar(250) NULL
,[Lvl3] nvarchar(250) NULL
,[Lvl2] nvarchar(250) NULL
,[Lvl1] nvarchar(250) NULL
,[Element ID] nvarchar(250) NULL
,[Activiteit] nvarchar(250) NULL
,[Doel] nvarchar(250) NULL
,[Activiteitsnummer] nvarchar(250) NULL
,[Begindatum/-tijd] date NULL
,[Einddatum/-tijd] date NULL
,[Status projectfase] nvarchar(250) NULL
,[Afsluitingsdatum/-tijd] date NULL
,[Projectactiviteit] nvarchar(250) NULL
,[VerantwoordelijkeId] nvarchar(250) NULL
,[Verantwoordelijke] nvarchar(250) NULL
,[Categorie] nvarchar(250) NULL
,[Prioriteit] nvarchar(250) NULL
,[DataAreaID] nvarchar(250) NULL
);
WITH cteP AS (
SELECT
[NAME]
,ELEMENTNUMBER
,PARENTELEMENTNUMBER
,PathID = CAST(ELEMENTNUMBER AS VARCHAR(MAX))
,DATAAREAID
FROM HIERARCHYTREETABLE
WHERE NULLIF(PARENTELEMENTNUMBER, '') IS NULL
UNION All
SELECT
[NAME] = r.[NAME]
,ELEMENTNUMBER = r.ELEMENTNUMBER
,PARENTELEMENTNUMBER = r.PARENTELEMENTNUMBER
,PathID = p.PathID+CONCAT(',',CAST(r.ELEMENTNUMBER AS VARCHAR(MAX)))
,DATAAREAID = r.DATAAREAID
FROM HIERARCHYTREETABLE r
JOIN cteP p ON r.PARENTELEMENTNUMBER = p.ELEMENTNUMBER
AND r.DATAAREAID = p.DATAAREAID
)
,
cteP2 AS (
SELECT
B.Lvl3
,B.Lvl2
,B.Lvl1
,HIERARCHYTREETABLE.ELEMENTNUMBER AS 'Element ID'
,HIERARCHYTREETABLE.[NAME] AS 'Activiteit'
,SMMACTIVITIES.PURPOSE AS 'Doel'
,SMMACTIVITIES.ACTIVITYNUMBER AS 'Activiteitsnummer'
,SMMACTIVITIES.STARTDATETIME AS 'Begindatum/-tijd'
,NULLIF(SMMACTIVITIES.ENDDATETIME, '') AS 'Einddatum/-tijd'
,CASE WHEN SMMACTIVITIES.CLOSED = 1 THEN 'Gesloten' ELSE 'Open' END AS 'Status projectfase'
,NULLIF(SMMACTIVITIES.ACTUALENDDATETIME, '') AS 'Afsluitingsdatum/-tijd'
,SMMACTIVITIES.PROJACTID AS 'Projectactiviteit'
,SMMACTIVITIES.RESPONSIBLEEMPLOYEE AS 'VerantwoordelijkeId'
,DIRPARTYTABLE.[NAME] AS 'Verantwoordelijke'
,CASE SMMACTIVITIES.CATEGORY
WHEN 0 THEN 'Afspraak'
WHEN 1 THEN 'Taak'
WHEN 2 THEN 'Actie'
WHEN 3 THEN 'Melding'
END AS Categorie
,CASE SMMACTIVITIES.TASKPRIORITY
WHEN 0 THEN 'Normaal'
WHEN 1 THEN 'Laag'
WHEN 2 THEN 'Hoog'
END AS Prioriteit
,HIERARCHYTREETABLE.DATAAREAID
FROM HIERARCHYTREETABLE
LEFT JOIN cteP
ON cteP.ELEMENTNUMBER = HIERARCHYTREETABLE.ELEMENTNUMBER
AND cteP.DATAAREAID = HIERARCHYTREETABLE.DATAAREAID
CROSS Apply (
SELECT Lvl1 = xDim.value('/x[3]','varchar(50)')
,Lvl2 = xDim.value('/x[2]','varchar(50)')
,Lvl3 = xDim.value('/x[1]','varchar(50)')
,Lvl4 = xDim.value('/x[4]','varchar(50)')
FROM ( VALUES (CAST('<x>' + REPLACE(PathID,',','</x><x>')+'</x>' AS xml))) B(xDim)
) B
LEFT JOIN SMMACTIVITIES
ON SMMACTIVITIES.RECID = HIERARCHYTREETABLE.REFRECID
AND SMMACTIVITIES.DATAAREAID = HIERARCHYTREETABLE.DATAAREAID
LEFT JOIN EMPLTABLE
ON SMMACTIVITIES.RESPONSIBLEEMPLOYEE = EMPLTABLE.EMPLID
LEFT JOIN DIRPARTYTABLE
ON DIRPARTYTABLE.PARTYID = EMPLTABLE.PARTYID
AND DIRPARTYTABLE.DATAAREAID = EMPLTABLE.DATAAREAID
)
INSERT INTO #Tempproj(
[ProjKey],Lvl1,Lvl2,Lvl3,[Element ID]
,Activiteit,Doel,Activiteitsnummer,[Begindatum/-tijd],[Einddatum/-tijd],[Status projectfase]
,[Afsluitingsdatum/-tijd],Projectactiviteit,VerantwoordelijkeId
,Verantwoordelijke,Categorie,Prioriteit,DataAreaID)
SELECT
PROJTABLE.PROJID + '-' + PROJTABLE.DATAAREAID AS 'ProjKey'
,cteP2.*
FROM CteP2
LEFT JOIN HIERARCHYTREETABLE
ON HIERARCHYTREETABLE.ELEMENTNUMBER = CteP2.Lvl3
AND HIERARCHYTREETABLE.DATAAREAID = CteP2.DataAreaId
LEFT JOIN PROJTABLE
ON PROJTABLE.PROJID = HIERARCHYTREETABLE.[NAME]
AND PROJTABLE.DATAAREAID = HIERARCHYTREETABLE.DATAAREAID
WHERE Activiteit NOT LIKE 'MLD%'
AND Activiteit NOT LIKE 'O-%'
AND Activiteit NOT LIKE 'OTR%'
AND SUBSTRING(Activiteit, 1, 1) NOT IN ('1','2','3','4','5','6','7','8','9');
DECLARE #Columns as VARCHAR(MAX)
SELECT
#Columns = COALESCE(#Columns + ', ','') + QUOTENAME(Activiteit)
FROM
(SELECT DISTINCT Activiteit FROM #Tempproj
) AS B
ORDER BY B.Activiteit
DECLARE #SQL as VARCHAR(MAX)
SET #SQL = 'SELECT ProjKey, ' + #Columns + '
FROM
(
SELECT T.ProjKey,
T.Activiteit,
T.[Begindatum/-tijd]
FROM #Tempproj T
) as PivotData
PIVOT
(
max([Begindatum/-tijd])
FOR Activiteit IN (' + #Columns + ')
) AS PivotResult
ORDER BY ProjKey'
EXEC (#SQL)
Then when I run the query, it says that I haven't declared my #Tempproj table yet.
Now I see that I mix up SQL with Dynamic SQL, but I don't know how to fix this giant statement into this.
Any suggestions what I could do here?
EDIT: Table Variable replaced with an actual Temporary Table.