I am trying to update a custom number field of a list of custom objects

I am trying to update a custom number field of a list of custom objects. I keep receiving the error Error: Compile Error: Field is not writeable: jstcl__TG_Timesheet__c.Period__c at line 12 column 22. However, this varible isn't the one being written over, I'm just reading what it's value is on the if statement.

This is what I have so far:

public class CamstimecardEndDateAudit{
public static void auditTimeCards(){
    List<jstcl__TG_Timesheet__c> myList = new List<jstcl__TG_Timesheet__c>();

     myList = [SELECT jstcl__Placement__r.Name, Period__c, TCEndDateAuditB__c
                            FROM jstcl__TG_Timesheet__c 
                            WHERE jstcl__Placement__r.ts2__Status__c IN ('Active') 
                             AND jstcl__Week_Ending__c = LAST_N_DAYS:15
                             AND jstcl__Status__c = 'Pending'];
    
    for(integer i=0 ; i < myList.size(); i++) {
        if(myList[i].Period__c = 'Weekly Split'){
    myList[i].TCEndDateAuditB__c = myList[i].TCEndDateAuditB__c + 0.5;}
       else{
    myList[i].TCEndDateAuditB__c = myList[i].TCEndDateAuditB__c + 1;
    }
    }
    
  update myList;
}}