While creating an accounting method, you are unable to select a journal entry rule set...

The accounting method is either not attached to any chart of accounts or is attached to another chart of accounts than the journal entry rule set

Journal line rules assigned to the journal entry rule set are not attached to the same chart of accounts to which the accounting method is attached

Description rules assigned to the journal entry rule set are not attached to the same chart of accounts to which the accounting method is attached

Supporting references assigned to the journal entry rule set are not attached to the same chart of accounts to which the accounting method is attached