Rangers fielded players who were ineligible to play.

SFA registration

7. No player shall be entitled to play Association Football in Scotland under the auspices
of the Scottish FA unless they are registered with the Scottish FA in accordance with
these Procedures.

also

4.1.6 All payments to be made to a Player relating to his playing activities must be clearly
recorded upon the relevant written agreement . No other payments for his playing
activities may be made to a Player via a third party or otherwise.
The players were clearly in breach of 4.1.6 so, as per rule 7, were not allowed to play