It reads as "player shan't play unless club holds registration (and eligible) between stated time periods", then just says apropos of nothing the phrases "either included on the Squad List; or Under 21 Player" which doesn't mean anything without qualifying why those two clauses relate to the previous text. There should be an "and" after "Season" as one of the U.1.1 and U.1.2 statements is a required conditions for the player, and without the extra "and" they're just some words not related to the previous statement.
Arguably using this typo a legal team could defend a club fielding a registered player who isn't named on the squad list.
Unless Im reading it wrong, the bit in bold is the addition to the first part. The first part non-explicitly refers to non-league matches and games before the close of the first transfer window; the second part then goes on to say that once the window is closed then they need to be on the "list" or a u21.