As Spaceman Spiff, Yorkshire Blue, and Napster have already pointed out.
From The EFL rules and regulations
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46 Registration Forms
46.1 In order to register a Player the prescribed contract, registration form and/or transfer form signed on behalf of the Club by either the Chairman, club secretary or any duly appointed signatory must be sent to The League within five days of the Player's signature and are subject to the approval of The League. The receipt by the registering Club of confirmation of registration shall determine the eligibility (and the effective date of eligibility) of the Player to play in League Matches.
Guidance
For the purposes of this Regulation, any other ‘duly appointed signatory’ (otherwise known as ‘Authorised Signatories’) must submit to the Owners’ and Directors’ Test.
Note, Regulation 63.3 provides that contracts can be signed by (a) the Chairman alone or (b) any other director and one other authorised signatory.
The EFL Board has previously confirmed (Season 2010/11) that irrespective of whether or not a Club is formally embargoed, the EFL reserves the right under this Regulation to decline the registration of a player in the event that the Club concerned has demonstrated that it is unable to meet its existing financial obligations. Whilst most liabilities are already specifically covered under the Regulations, there can be cases where Clubs have demonstrate an inability to meet their other debts as and when they fall due which are not directly covered by the Regulations, and if those Clubs were permitted to continue unfettered in the transfer market this could undermine the integrity of our competitions and the public reputation of the EFL as a whole. The prospect of declining registrations outside of an embargo is only seen as an exceptional event and will only be deployed when the EFL deems that the circumstances are unusual enough to require this course of action.
53 Non-Payment to Players
53.1 If following written notification from the Professional Footballers’ Association, a Club is found to be in default of any payment due in accordance with a Player's contract or payment in consequence of termination of such contract, The League may place an embargo on any transfer of any registration by such Club until the payment is made.
Guidance
Clubs in default of payments to players will ordinarily be subject to an embargo. The EFL does recognise there are cases where payment may well be in dispute, but expects the matter to have been referred to a Player Related Dispute Commission in accordance with Regulation 73 for resolution to avoid the imposition of the embargo.
The EFL Board has previously confirmed (Season 2010/11) that irrespective of whether or not a Club is formally embargoed, the EFL reserves the right under this Regulation to decline the registration of a player in the event that the Club concerned has demonstrated that it is unable to meet its existing financial obligations.
EFL Statement 20th Jan 2020
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Southend United Charged with Misconduct
Southend United has been charged with misconduct for non-payment of wages for failing to pay a number of players on the applicable payment due dates in December 2019.
Whilst outstanding payments were made in the New Year, the Club has now been referred to a Disciplinary Commission for failing to adhere to the terms of the players’ contracts.
As this matter is now subject to proceedings, the EFL will be making no further comment at this time.
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Now I wonder why Southend United have still not signed any players?