For create round

Hello!

you need to use request core/search_items to search element avl_route with flag base flag and schedules to get t ItemId(route Id), and  sh (schedule Id)
When you create new round, you just specify Id (round Id )= 0 , after creating system will be automatically set up index number for new round, you'll see real id in response after create round.

You need to specify "propName":"sys_name" ,  "sortType":"sys_name", "propValueMask":"name_of_route"), "flags":"1205" - base flag + schedules flags, please look here

Hello!

The error 4 can mean that there is not correct parameters (names, values, types, parameters lack) as well as syntax errors (missing or special symbols like , : }  ect, or spaces in JSON).
You can look through this example. The route with id = 2357 ("itemId") is already created , the schedule (id = 2) also is created
put:
round id is 0 - "id"
"sh":2- schedule id above
"u":2174 - unit appointed to this round
"at":0 - activation time, 0 means that the system will activate round automatically when arrival time of 1st point is coming. Round activation means that the system will start to control visits to routes points
"vt":1623057394 - (Jun 07 2021 09:16:34 GMT+0000)
"tz":134228528 - user's time zone, GMT+3
"vp":3600 -  validation period of round, in sec, 3600 sec (24 h) - default value. So after 24 h from 'vt' time ( Jun 07 2021 09:16:34 GMT+0 ) the round will be interrupted automatically
(the system won't control it)

https://hst-api.wialon.com/wialon/ajax.html?svc=route/update_round&sid=0271315eb061fa7baae1b951

params: {"n":"new round1","d":"","u":2174,"at":0,"vt":1623057394,"vp":3600,"sh":2,"cu":[],"f":2,"tz":134228528,"itemId":2357,"id":0,"callMode":"create"}