Because you SQL script(which you have posted here) doesn't contain the things
dateEventDate & timeStartTime.
OR
you forgot to put the single quote for CONVERT() things.
below is working one
declare @sql varchar(1000)
set @sql='select * from tblEvents where '+'CONVERT (varchar(24),dateEventDate,113)'+ '+'+''' '''+'+'+ 'CONVERT(varchar(20),timeStartTime)'+'>='+'(CONVERT(varchar(24),getdate(),113))'
exec (@sql)