Based on your cash balance the board sets a "Daily wage allowance". If your current wages paid per day for your squad is higher than what the board allows, the board will block sales where to need to pay more than 25% of your current cash balance.

 

You can increase the wage budget by getting more cash. Selling a player will help. This will both reduce your current cash balance and decrease your daily wage bill - and make room for new signings.

 

The board wants to secure you have a financially healthy club. If your wages are too high compared to your cash balance they will block purchases of new players. To cope with this you must increase your cash balance or lower your wage bill. It is done so you do not over-finance your club and end up bankrupt soon, being forced to sell. It is financial fair play and the rules are the same for all clubs - so everyone must work with these rules. So build your club long term with solid finances and you will never have problems with wages. You can earn cash via player sales, cash injection, playing matches, signing sponsor deals and collection reward boxes via various things like achievements, fan duels, quests and world tour.