If you can't save your quick reply, check to make sure it follows all of the following formatting rules.

• The maximum number of stored quick replies allowed is 50.
• The maximum length of a quick reply shortcut is 25 characters.
• Shortcuts can’t include spaces.
• All shortcuts must begin with a forward slash /.
• The maximum number of keywords allowed per quick reply is three.
• Keywords may not contain spaces or any non-alphabetic, non-numeric characters.
• The maximum length of a single keyword is 15 characters.

Related resources:

How to use quick replies: Android | iPhone