Cannot see the face but that appears to me to be a board screwed into the supports projecting from the wall. Remove the screws carefully and the board should come off. It may be 8 or 10' long so you may want an extra pair of hands or two.

The cover board nailed to the supports projecting from the wall. I thought, may be somebody seen this design and there is another way to remove the cover

Do you want to replace the fixture or light tube.

Light tube have pins on each end, use two hand twist/spin/turn the tube, you will notice when you released the tube from the fixture.

I need to replace the light fixtures, not lamps

" I need to replace the light fixtures, not lamps "

Then the cover board needs to be removed to get at the attachments for the light. You will also need the the cover off to install new lighting.