In case if you see there is an error in red which states company is not open in tally and sync is not happening in mobile app then follow the below steps for the resolution :
Check if company is open in tally or not and also check if ODBC port is there or not.
If company is not open :
Then open the company in tally and wait to get it sync automatically.
No ODBC port :
Then follow the below steps :
Press F12-)Advance Configuration-)ODBC Configuration.
Need to update below details :
Tally.ERP 9 is acting as : Server
Enable ODBC server : Yes
Port : 9000
Then Press Ctrl A to save the setting.