I work for the Supervisor of Assessments office in Lawrence County and I want to run for Lawrence Township Assessor is that legal ?

My best recommendation would be to contact the local State's attorney office to confirm if this is OK.  I do not see why you cannot run for this office, though you may be required to take a leave of absence.  Even if this is proper, you need to be very careful that any campaign work you engage in is 100% separate from your job.  That means everything you do must be done after normal work hours and you cannot use any resources from your county office: do not use the copy machine, the fax, your county email address, no pens and pencils purchased by the county, etc