Can't Reconcile

I use the manual logging in Goodbudget. Everytime I to to reconcile my account I get the following message:
There are too many transactions between your last Reconciliation and the Statement Date you’ve entered. To reconcile fewer transactions at a time, change your Statement Date and Balance to reconcile your Account one month at a time, starting from the earliest unreconciled bank statement.
I enter the beginning balance and only reconcile for one month. Am I doing something wrong? Do I have to have my bank account synced to reconcile?

If you’re only doing one month at a time you shouldn’t be seeing the “too many transactions” message unless you have a particularly busy account. Did you happen to import a lot of older transactions you’re not reconciling? If so, you may need to reconcile those separately, but that would take some tweaking to make your current reconciliation work.
I’m not sure what the cutoff is for that message (nor why it exists at all) but I’ve only ever seen it when I have multiple pages of transactions showing.
You definitely do NOT have to sync your accounts to use the reconcile feature.