Hi Gracia,

You trigger needs to be bulkified:

1. Don't use the SOQL Queries , DML Statements inside for loop.
2. Don't use the hardcoded record id's in trigger. use the profile name to query the user in below line.
    List<User> uList = [SELECT Id FROM User Where IsActive = true and ProfileId = '00e4x000000NCX0AAO']; change to below line
    List<User> uList =  [ SELECT Id FROM User Where IsActive = true and Profile.Name = 'System Administrator' ];
3. In test class don't use the (SeeAllData=true). Instead of that create test data in test class.
4. Use after contexts in trigger while you performing the actions in related objects.

Please try the below trigger:
 

trigger SLInsertOppTeam on Opportunity (after insert, after update) {
    Map<Id,List<OpportunityTeamMember>> oppMemberMap = new Map<Id,List<OpportunityTeamMember>>();
    set<Id> oppIds = new set<Id>();
    List<OpportunityTeamMember> otmList = new List<OpportunityTeamMember>();
    List<User> uList = [SELECT Id FROM User Where IsActive = true and ProfileId = '00e4x000000NCX0AAO'];
    for(Opportunity opp : trigger.New){
        if(opp.StageName == 'A' && opp.is_new__c == false)
        {
            oppIds.add(opp.Id);
        }
    }
    
    for ( OpportunityTeamMember member : [SELECT Id,OpportunityId,TeamMemberRole FROM OpportunityTeamMember Where OpportunityId IN: oppIds AND TeamMemberRole = : 'Analist']){
        if(!oppMemberMap.containsKey(member.OpportunityId)){
            oppMemberMap.put(member.OpportunityId, new List<OpportunityTeamMember>{member});
        }
        else{
            oppMemberMap.get(member.OpportunityId).add(member);
        }
    }
    
    for(Opportunity opp : trigger.New){
        if(opp.StageName == 'A' && opp.is_new__c == false)
        {
                List<OpportunityTeamMember> oppMemberList = new List<OpportunityTeamMember>();
                if(oppMemberMap.containsKey(opp.Id)){
                    oppMemberList = oppMemberMap.get(opp.Id);
                }
                if(oppMemberList.size() == 0 )
                {
                    OpportunityTeamMember OTM = new OpportunityTeamMember();
                    OTM.OpportunityId = opp.Id;
                    OTM.TeamMemberRole = 'Analist';
                    OTM.OpportunityAccessLevel = 'Edit';
                    if(uList.size() > 0){
                        Integer Ran = (integer)(Math.random() * (uList.size()));
                        OTM.UserId = uList[Ran].id;
                    }
                    otmList.add(OTM);
                }
        }
    }
    
    if(!otmList.isEmpty()){
        insert otmList;
    }
}

Test Class:

I assumed opp.is_new__c field become as false in opp update so i have updated the opp in below test class. Otherwise you can remove the update statement below.

@isTest
public class SLInsertOppTeamTest {
	static testmethod void testSLInsertOppTeam(){
        Account testAccount = new Account();
        testAccount.Name='Test Account record' ;
        insert testAccount;
        
        // Add if there is any other Mandatory fields in Opportunity.
        Opportunity opp = new Opportunity();
        opp.name='oppname';
        opp.accountid=testAccount.id;
        opp.CloseDate=system.today().addDays(30);
        opp.StageName='A';
		opp.is_new__c == false;  // If this a formula field then remove it. 
		insert opp;
        test.startTest();
			opp.name='testoppname';
			update opp;
        test.stopTest();
    }
}

Thanks,
Maharajan.C