Can I migrate Demo User mode data(In txt file) to sqlserver
No. TXT is only for testing SambaPOS. It is very limited and cannot be used in production.
Install SQL Express and start again.
The following is a tutorial on installing SQL Express
http://sambapos.org/wiki/doku.php/en/installing_sql_express_2012_for_sambapos_v3
John you mean have to enter all the data again?