Missing feature, Details: Installing this package requires the following feature and its associated permissions: Permission Set Groups

The problem for me was that the status of the permission set group (PSG) changed after adding the PSG-Component and second Permission Set. I had to assign the PSG first before inserting new components. This fixed my problem:

//create user with permissions
public static User createUserPermissionDataModel() {
    //permission set
    PermissionSet ps = new PermissionSet(
         Name = 'TestPermissionSet',
         Label = 'Test Permission Set'
    );
    insert ps;
    //permission set group
    PermissionSetGroup psg = new PermissionSetGroup(
        DeveloperName = 'TestPermissionSetGroup',
        MasterLabel = 'Test Permission Set Group'
    );
    insert psg;
    //role
    UserRole role = new UserRole(
         Name = 'Developer',
         DeveloperName = 'Developer',
         CaseAccessForAccountOwner = 'Edit',
         OpportunityAccessForAccountOwner = 'Edit'
    );
    insert role;
    //user
    User permUser = new User(
         FirstName = 'test',
         LastName = 'user',
         Username = '[email protected]',
         Email = '[email protected]',
         Alias = 'testuser',
         TimeZoneSidKey = 'Europe/Berlin',
         LocaleSidKey = 'en_US',
         LanguageLocaleKey = 'en_US',
         EmailEncodingKey = 'UTF-8',
         profileId = [SELECT Id FROM Profile WHERE Name = 'Standard User'].Id,
         UserRoleId = role.Id
    );
    insert permUser;
    //permission set group assignment
    PermissionSetAssignment psga = new PermissionSetAssignment(
         AssigneeId = permUser.Id,
         PermissionSetGroupId = psg.Id
    );
    insert psga;
    
    //permission set for group
    PermissionSet ps2 = new PermissionSet(
        Name = 'TestPermissionSetForGroup',
        Label = 'Test Permission Set For Group'
    );
    insert ps2;
    //permission set group component
    PermissionSetGroupComponent psgc = new PermissionSetGroupComponent(
         PermissionSetGroupId = psg.Id,
         PermissionSetId = ps2.Id
    );
    insert psgc;
    //permission set assignment
    PermissionSetAssignment psa = new PermissionSetAssignment(
        AssigneeId = permUser.Id,
        PermissionSetId = ps.Id
    );
    insert psa;

    return permUser;
}
@isTest
private static void testGetUserPerm() {
    Test.startTest();
    List<PermissionHelper.PermInfos> permInfos = UserPermissionController.getUserPerm(testUser.Id);
    Test.stopTest();
    System.assertNotEquals(null, permInfos);
}

As per docx, Permission set group is available for the users whom having the system permission Manage Profiles and Permission Sets in his profile.

So can you please check whether you're current profile having this permission?

If yes, Please log a case to Salesforce because this feature is already released on Spring'20 Release.

Thanks.