I am having this problem too. to reproduce the error:
1. install version 2.0.x (i m using 2.0.4 and 2.0.7 to be exact).
2. upgrade to 2.1
3. follow the upgrade procedure here: https://frontaccounting.com/wbt/pages/download/update-guide.php
4. login to frontaccouting as admin
5. click on "Direct Invoice"
------------------------- Error Message ----------------------------------------------------------
DATABASE ERROR : could not get POS definition
error code : 1064
error message : You have an error in your SQL syntax near 'AND pos.pos_location=loc.loc_code AND pos.pos_account=acc.id' at line 1
sql that failed was : SELECT pos.*, loc.location_name, acc.bank_account_name, acc.account_code FROM 0_sales_pos as pos,0_locations as loc,0_bank_accounts as acc WHERE pos.id= AND pos.pos_location=loc.loc_code AND pos.pos_account=acc.id
------------------------- Error Message ----------------------------------------------------------
Hope can help.
TQ