Mysql getting first and last value from timestamp column
full answer required more understanding of all tables structure, but You need start from debug Your inner query:

select 
    distinct tasks_id,
    complete, 
    createdate,  
    min(createdate) as firstEntry, 
    max(createdate) as lastEntry 
from tasklogs 
WHERE complete in (90) and createdate &gt; '2018-08-25' 
GROUP BY createdate


it wrong (it wrong and with sql_mode='', and total), with GROUP BY createdate, all 3:


createdate
min(createdate)
max(createdate)


would be the same

as variant - may be You need GROUP BY tasks_id, and include only 3 column:


task_id
min(createdate)
max(createdate)