Invoice register posting

When trying to post A/P I rec'd the following "Warning" message:  "The A/P posting date falls into a prior G/L period. You must change the accounting date or re-open the period in G/L."

Since I am new to Sage 100, what does this mean?  A co-worked was inputting budget info for the new fiscal year (2017) just before I finished entering A/P and tried to post/print the Invoice register.  Any connection?