>If I wrote
>
>on error 1881 = messagebox ("bad data")

this one is a logical expression so it compare 1881 with the result of messagebox.. it will result always .f.
>
>it works OK, but if I wrote
>
>on error 1881 do form error1
>
>It gives sintax error
>
>What can be wrong ?

try:

m.tipe_err = 1 && serious one
m.tipe_err = 2 && not so serious
m.tipe_err = 3 && just warning

On ERROR DO errorhandler WITH MESSAGE(), ERROR(), LINENO(), m.tipe_err, PROGRAM()

Function errorhandler
    Parameters m.messg, m.error_no, m.lineno, m.code, m.programid
    #if  version(2)=2
         SET STEP ON
    #endif
    If  m.error_no = 1881
        do form error1
        Return
    Endif

HTH