It has to be a backslash = \, not a slash = /
And in common the default instance name is SQLEXPRESS, not sqlserverexpress. So try it with
Using Sage300 app online from Seneca College and SQL Management 2014 installed on my laptop! Last week connected perfectly and did an assignment! This week I can connect to Sage online App, but when I start SQL Management it says Error26 can not connect to database! We input the path of database manually as: Mylaptopname/sqlserverexpress. I didn’t do any changes since last week so please help me to solve my problem! Thank you in advance for your time.