I had same problem.

I’ll leave the information for someone who might have same issue in the future.

First,  I tried to connect my server by other app (like FTP manager) and made sure all the setting information is correct. (Host, User, pw, etc) Then I realized that the setting to connect my server was correct.

So I went back to Textastic and deleted the current setting and set a new connection with the correct setting information. That made me connect to the server successfully!

I think it’s really beginner problem but wishing above information will be someone’s help.