[modeller_usage] Error: No atoms were read from the specified input PDB file, since the starting residue number and/or chain id in MODEL_SEGMENT

Dear Users

Hello

I am trying to build a model of a protein sequence where I have structures of two domains of the same protein as an input. One structure is collected from PDB. another is simulation generated. I got the following error:

Traceback (most recent call last):
  File "align1.py", line 7, in <module>
    mdl = Model(env, file=pdb, model_segment=('FIRST:A', 'LAST:A'))
  File "/usr/lib/python3.8/dist-packages/modeller/model.py", line 101, in __init__
    self.read(**vars)
  File "/usr/lib/python3.8/dist-packages/modeller/model.py", line 151, in read
    return _modeller.mod_model_read2(self.modpt, io.modpt,
_modeller.ModellerError: rdpdb___303E> No atoms were read from the specified input PDB file, since the starting residue number and/or chain id in MODEL_SEGMENT (or the alignment file header) was not found; requested starting position: residue number " FIRST", chain " A"; atom file name:  A1.pdb

The simulation generated structure file looks like following:
ATOM      1  N   MET X   1      43.720  45.860  63.480  0.00  0.00            
ATOM      2  H1  MET X   1      44.460  45.570  64.150  0.00  0.00            
ATOM      3  H2  MET X   1      42.750  45.730  63.840  0.00  0.00            
ATOM      4  H3  MET X   1      43.890  45.180  62.720  0.00  0.00            
ATOM      5  CA  MET X   1      43.900  47.280  63.110  0.00  0.00            
ATOM      6  HA  MET X   1      43.520  47.850  63.950  0.00  0.00            
ATOM      7  CB  MET X   1      45.340  47.790  62.980  0.00  0.00            
ATOM      8  HB1 MET X   1      45.450  48.870  62.740  0.00  0.00            
ATOM      9  HB2 MET X   1      45.800  47.630  63.980  0.00  0.00            
ATOM     10  CG  MET X   1      46.250  47.060  61.990  0.00  0.00            
ATOM     11  HG1 MET X   1      47.240  47.350  62.410  0.00  0.00            
ATOM     12  HG2 MET X   1      46.180  45.950  62.040  0.00  0.00            
ATOM     13  SD  MET X   1      46.260  47.620  60.260  0.00  0.00            
ATOM     14  CE  MET X   1      47.960  47.000  60.080  0.00  0.00            
ATOM     15  HE1 MET X   1      47.960  45.900  60.270  0.00  0.00            
ATOM     16  HE2 MET X   1      48.350  47.180  59.060  0.00  0.00            
ATOM     17  HE3 MET X   1      48.640  47.540  60.770  0.00  0.00            
ATOM     18  C   MET X   1      43.020  47.620  61.960  0.00  0.00            
ATOM     19  O   MET X   1      42.650  46.780  61.140  0.00  0.00            
ATOM     20  N   ALA X   2      42.710  48.920  61.800  0.00  0.00            
ATOM     21  HN  ALA X   2      43.190  49.580  62.370  0.00  0.00            
ATOM     22  CA  ALA X   2      42.060  49.460  60.650  0.00  0.00            
ATOM     23  HA  ALA X   2      41.530  48.650  60.160  0.00  0.00            
ATOM     24  CB  ALA X   2      41.310  50.730  61.090  0.00  0.00            
ATOM     25  HB1 ALA X   2      41.940  51.410  61.700  0.00  0.00     

The code used is the following:

from modeller import *

env = environ()
aln = alignment(env)
env.io.hetatm = True
for (pdb, chain) in (('UP1', 'A'), ('A1', 'X')):
    mdl = Model(env, file=pdb, model_segment=('FIRST:A', 'LAST:X'))
    aln.append_model(mdl, atom_files=pdb, align_codes=pdb)
aln.append(file='query1.ali', align_codes='query')
aln.malign()
aln.write(file='querytemplate_new.ali', alignment_format='PIR')
aln.write(file='querytemplate_new.pap', alignment_format='PAP')

Any help regarding how to solve this error will be great.

Thanks

Ira