Your environment appears to be running a different PHP version ( 5.6.40-0+deb8u11 ) for the System Cron than the currently loaded PHP version ( 7.3.21-1+0~20200807.66+debian10~1.gbp18a1c2 ). This may cause issues running your System Cron.
@MaKiSpAiN maybe post the full command you are trying to run via cron, so we can help you better?
@eris already pointed out most likely you could simply specify which php version your cron command should use instead of a plain php -xy dosomething so that it matches the version you set your clients web domain to.