Subquery returned more than 1 value. This is not permitted when the subquery follows

[fubaba]

hi, i execute a store procedure got
Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.' message

is there any one help with these codes below
.......
SET @strSQL = (@strSQL + ' AND p.PropertyID NOT IN ( ' + (SELECT DISTINCT a.propertyID
FROM propertyAvailab ility a
WHERE a.date >= CONVERT(varchar (10), getDate(), 103)
AND a.date <= CONVERT(varchar (10), getDate(), 103) )+')')
..........