i've come so far in thieves guild that i am supposed to get a job involving some guy Erikur in solitude but, I made a mistake like 10 lvls ago by killing him... Does someone know a way to get around this obstacle??? im very happy for all the help i can get.

if he is involved in a quest you shouldnt have been able to kill him.

If your on pc you can try using console to revive him.... All I have in mind... or you can just skip the quest. Because i don't think theres much to do once you killed him.