There could be two possible reasons for not being able to send Waves / Personalized messages:

1. You have crossed your daily limit of 75 messages per day: 
In this case, you can come back again the next day and send more messages. Please note, replying to messages is unlimited. 

2. Your personalized message might contain words or phrases which have been detected by our filters which could be spammy or abusive. In such circumstances, we recommend you read our Code of Conduct to know what is acceptable content