core_version_requirement

 cf :https://www.drupal.org/node/3070687  

This version is not compatible with Drupal 9.0.0 and should be replaced.

Machine name: fahp_synthesis

Requires:

• Drupal Core (8.x) (incompatible with version 9.0.0)

Edit my_custom_module.info.yml:

change

core: 8.x

to

core: 8.x
core_version_requirement: ^8 || ^9

for drupal 9.x