@Tamilvanan T 

you can use scripted approvals, combine all the users from list collector and then set

Scripted Approvals in Flow Designer with Flow Variables 

or you can use Lookup Records on sys_user with condition as

SysId IS ONE OF Get Catalog Variable->List Collector

Then use Flow variable to append the user sysId and then use Ask for approval action and use this flow variable to set the approval users

Note: Don't use ask for approval multiple times i.e. don't loop through it, use Ask for Approval only once and set the approvers

If my response helped please mark it correct and close the thread so that it benefits future readers.