I've to create an automation process to check that no new opportunities has been created for an account in past 12 months and update the account field based on that. Tried process builder, but it doesn't seem to work.
Tricky
A flow/workflow/process builder needs some triggering condition to fire. If an account was created 5 years ago, not updated since, haven't had any opportunities - it will not trigger any flows until somebody touches it.
And even if you somehow to manage to make a time-based workflow for example (to enqueue making a Task 1 year from now if there are no Opps by then) - it'll "queue" actions only from the moment it was created, it will not retroactively tag old unused accounts.
The time-based actions suck a bit. Say you made it work, it enqueued some future tasks/field updates/whatevers. Then you realise you need to exclude Accounts of certain record type from it. You need to deactivate the workflow/flow to do it - and deactivation wipes the enqueued actions out. So you'd need to save your changes and somehow "touch" all accounts again so they're checked again.
Does it have to be a field on Account? Can it be just a report (which you could make a reporting snapshot of if needed)? You could embed a report on account layout right? A query? Worst case some apex nightly job that runs and tags the accounts? It would dutifully run through them all and set/clear your helper field, easy to change (well, for a developer).
SELECT Id, Name
FROM Account
WHERE Id NOT IN (SELECT AccountId FROM Opportunity WHERE CreatedDate = LAST_N_DAYS:365)
Reporting way would be "cross filter": https://salesforce.vidyard.com/watch/aQ6RWvyPmFNP44brnAp8tf, https://help.salesforce.com/s/articleView?id=sf.reports_cross_filters.htm&type=5