This is because you don't have a group by but you do have aggregate data. It is a bit difficult to say what you need to do here but as a guess you might try putting the aggregate columns and the primary key into a cte then you can join to the cte from your query. Something like this maybe?

with AggregateData as

(

select UserID,

MAX(Case when Ugedag = 'Mandag' then frakl + ' - ' + tilkl else null end) as startSlutMandag,

MAX(Case when Ugedag = 'Tirsdag' then frakl + ' - ' + tilkl else null end) as startSlutTirsdag,

MAX(Case when Ugedag = 'Onsdag' then frakl + ' - ' + tilkl else null end) as startSlutOnsdag,

MAX(Case when Ugedag = 'Torsdag' then frakl + ' - ' + tilkl else null end) as startSlutTorsdag,

MAX(Case when Ugedag = 'Fredag' then frakl + ' - ' + tilkl else null end) as startSlutFredag

from example

group by UserId

)

SELECT FagNavn, adgangskrav, frakl, tilkl, Ugedag, holdId, fagKode, CONVERT(CHAR(10), StartDato, 105) AS startDate,

CONVERT(CHAR(10), slutDato, 105) AS endDate, afdeling, link, navn, deltagerBetaling, bt, niveau,

ad.startSlutMandag, ad.startSlutTirsdag, ad.startSlutOnsdag, ad.startSlutTorsdag, ad.startSlutFredag

from VisWebHoldSkema vwhs

join AggregateData ad on ad.UserId = vwhs.FagNavn --or whatever you would use to join here

ORDER BY afdeling