If you have done the same thing as this post: https://learn.microsoft.com/en-us/answers/questions/114601/azure-ad-sync-connect-issue-with-permission-error.html
Your best bet would be open a Support Case with Microsoft (https://azure.microsoft.com/support/?WT.mc_id=AZ-MVP-5004796#support-plans) - so they can look at the logs - and do a general health check.
Make sure you are running the latest version of Azure AD Connect V2 - https://learn.microsoft.com/azure/active-directory/hybrid/whatis-azure-ad-connect-v2?WT.mc_id=AZ-MVP-5004796 - as V1 of AAD Connect (due to SQL support) went out of support back in July.