Some questions
1. Why fpc tries to link obj file generated with tasm? Must it understand this format?
2. Heve you success to link obj file generated with tasm ever? How can this be done?

//compiled with fpc106
program test;
begin
  writeln('hi');
end.

listing:

C:\FPC106\Tests>ppc386 test.pas -Atasm
Target OS: Win32 for i386
Compiling test.pas
Assembling program
Turbo Assembler Version 4.1 Copyright (c) 1988, 1996 Borland International

Assembling file: test.sw
Error messages: None
Warning messages: None
Passes: 1
Remaining memory: 450k

Linking test.exe
test.ow: file not recognized: File format not recognized test.pas(3,4) Error: Error while linking
Closing script ppas.bat

C:\FPC106\Tests>tlink32 test.ow
Turbo Link Version 1.6.71.0 Copyright (c) 1993,1996 Borland International Fatal: Bad loc for fixupp in module 'test.ow' near file offset 243

Some questions 1. Why fpc tries to link obj file generated with tasm? Must it understand this format?

No, the linker expects COFF format object files and not OMF

2. Heve you success to link obj file generated with tasm ever? How can this be done?

Never tried. You need at least a linker that supports OMF. But linking a full program will be difficult as the loaders are written in GNU as style assembler