public void ReAssignTask(List<Task> taskList){
        //Initialize variables
        String tempContainer;
        Set<Id> taskOwners = new Set<Id>();
        Map<Id,Id> taskOwnerMap = new Map<Id,Id>();
        Map<Id,Id> taskParentMap = new Map<Id,Id>();
        Map<Id,String> userProfileMap = new Map<Id,String>();
        Map<Id,Id> vsrToSR = new Map<Id,Id>();
        Map<Id,Id> srToVSR = new Map<Id,Id>();
        
        //Get owners of the tasks
        for(Task loopTask : taskList){
            taskOwnerMap.put(loopTask.Id, loopTask.OwnerId);
            taskParentMap.put(loopTask.Id, loopTask.WhatId);
        }
        
        //Group tasks depending on the profile of the owners of the tasks
        for(User loopUser : [SELECT id, profile.name FROM User WHERE Id IN :taskOwnerMap.values()]){
            userProfileMap.put(loopUser.Id, loopUser.profile.name);
        }
        
        for(Veeva_Service_Request__c loopVSR : [SELECT Id, Service_Request_Id__c 
                                                FROM Veeva_Service_Request__c
                                                WHERE Id IN :taskParentMap.values()
                                                    OR Service_Request_Id__c IN :taskParentMap.values()]){
            //Get Service Request for Veeva Service Request
            srToVSR.put(loopVSR.Service_Request_Id__c, loopVSR.Id);
            //Get Veeva Service Request for Service Requests
            vsrToSR.put(loopVSR.Id, loopVSR.Service_Request_Id__c);
        }
        
        //Re-assign service request tasks to its veeva service request counterpart
        for(Task loopTask : taskList){
            if(loopTask.WhatId.getSobjectType().getDescribe().getName() != 'Case' && 
               loopTask.WhatId.getSobjectType().getDescribe().getName() != 'Veeva_Service_Request__c' &&
               (userProfileMap.get(loopTask.OwnerId).contains('NAM') || 
                userProfileMap.get(loopTask.OwnerId).contains('LSM'))){
                loopTask.addError('A task cannot be assigned to LSM/NAM users if it is not related to a Case.');
            }
            if((userProfileMap.get(taskOwnerMap.get(loopTask.Id)).toLowerCase()).contains('nam')){
                //Assign task from service request to veeva service request
                if(srToVSR.containsKey(loopTask.whatId)){
                    loopTask.Service_Request_LookUp__c = loopTask.whatId;
                    loopTask.whatId = srToVSR.get(loopTask.whatId);
                }
            }else if((userProfileMap.get(taskOwnerMap.get(loopTask.Id)).toLowerCase()).contains('lsm')){
                //Assign task from service request to veeva service request
                if(srToVSR.containsKey(loopTask.whatId)){
                    loopTask.Service_Request_LookUp__c = loopTask.whatId;
                    loopTask.whatId = srToVSR.get(loopTask.whatId);
                }
            }else{
                //Assign task from veeva service request to service request
                if(vsrToSR.containsKey(loopTask.whatId)){
                    loopTask.Veeva_Service_Request_LookUp__c = loopTask.whatId;
                    loopTask.whatId = vsrToSR.get(loopTask.whatId);
                }
            }
        }
    }
}

Please check it has been initialized.