Where? At the point where renderer called "import" already added (and
assignment to a variable "cell" works well)

Think of the template like it is a separate module, it needs its own
list of imports. Just accessing the passed-in variables will work
regardless of imports, but as soon as something is to be referenced by
name, it must be imported first.

A possible place would be right at the beginning of the block:

extends layout

block content
   - import xyz;
   h3 this is a form:
   form(method='get')
       fieldset
           legend General
               label(for='username') Username:
                   input( type='text', name='username', value='vasya22' )
                   p #{cell} // done
                   p #{answ[0,0].as!PGtext} // error