First some code cleanup for better flow. I also include modifications for you to try:

lcOldNear = set("near")
set near off   && see if the setting for NEAR is ON. if so turn it off.
select 0
use Scanfile
select 0
use Ecu  &&dump from  sql
index on alltrim(label) to z
select Scanfile     && new line of code
locate
do while .not. eof("Scanfile")
   lab = allt(data)
   select Ecu
   seek lab
   if .not. eof("Ecu")
      replace scanfile.acct with  acct  && ok here
   else
      replace scanfile.noun with 'UNKNOWN'   && NEVER happens, remains blank
   endif
   select Scanfile
   skip
enddo
select Scanfile
set filter to noun = ' '
browse
select Scanfile
replace all noun with 'UNKNOWN'
set filter to
browse           && ALIAS NOT FOUND!

set near &lcOldNear