There are 2 explanations for this:
The driver is at a higher level than the buying account;
There was a change on how the transfer market works a few months ago, so you can't buy any driver/staff that are outside the market (
Changelog)
Brief explanation from a fellow Super Moderator from another
thread:
Kevin B
It is no longer permissible to purchase a driver (or staff) for 3 tokens on the "Black Market", this is why you see the lock symbol.
A bit more detail...
The exception to this is if the driver/staff member has been employed by your team previously, in this scenario you can still buy for 3 tokens outside of an auction. The reason for the exception is that some managers wait for a contract to expire and then buy back to overcome the 10% wage inflation.
In order to employ a driver or staff who you have not employed before they must appear on the auction market, you then have a choice of bidding (with the risk of someone else outbidding you) or "Buy Now" for 9 tokens.
If a manager wants to donate a driver or staff member to another team they must choose "Sell to Auction" to ensure the staff member is listed on the auction and then the receiving manager can purchase for 9 tokens.
Only exceptions are ex-members of the team and driver/staff that were in your shortlist prior to the update (July).
Keep an eye out for another good driver in the market and good luck! ?