The error 'Please note an error occurred while synchronising your split/analyse changes' suggests that nominal or project information is being added via the Analyse function in Access Approve, and a setting in the accounts system is preventing it from being processed.
The most common cause is the use of Levels in the Dimensions database. A user without access to a supplier via Levels will not be able to approve an invoice where analysis information is entered.
In the first instance, check with your accounts team whether Levels are in place and if they could be preventing the process. If Levels are not in place, for help contacting us, see Get support in Access Approve.