Error Prompt: You are a not an administrator for this ledger. If you need this ledger to be connected, contact its administrator."

Suggested Answer:

We understand the problem, but unfortunately we cannot do anything with this as it occurs on MYOB side. 

MYOB has improved their security and it's required for user to re-connect their MYOB files. According to MYOB only user who has admin role can to that 

https://apisupport.myob.com/hc/en-us/articles/13052864214671-OAuth2-0-Authentication-Now-Requires-Admin-Access

MYOB link: https://www.myob.com/au/support/myob-practice/client-accounting/sync-ledger/troubleshooting-sync-issues?productview=Browser