Depending on your current setting for fiscal year end you may not be able to go back to 2017. If your fiscal year is the same as the calendar year and you are working in 2019, then you can only go back as far as January 1, 2018. Sage 50 only allows entries in the current fiscal year and the immediate prior year.

If you are trying to make entries that were missed you are best not doing them in 2017 as that would change the financial reporting when filing the 2017 tax returns. If the entries are the accountants adjusting year end entries then you could enter them on the first day of the next year (2018). When doing this all the entries with accounts 1xxx, 2xxx, or 3xxx are OK but the entry with accounts 4xxx or 5xxx should be made to the account in 3xxx for Retained Earnings Prior Periods.This will allow your Balance sheet to reflect the correct values as of the first of the new fiscal period. It will also allow the changes to be reflected through the 2018 year end and into the 2019 fiscal year. 

Be cautious and speak with your accountant for advice if the 2017 and / or the 2018 year end tax returns have been filed.