Create custom error
If you like to have a custom error message you need to add a "Custom Error Message" in your question.
Ofcourse you need to have some of answer control as well to display the error when the condition is wrong.
NOTE about Decimals Error
The "Input format" error is triggered from the backend, because the format for the question cannot hold the data (number of digits) entered.
The error message you can not override (when it has been allowed to pass through your answer control).
The "Error message" element, override the standard error messages that is generated when the value does not pass the answer control that you have used.
For example if your answer control just has "Answer required", a value like 1234.6789 passes this control, and the backend message is then triggered. However if you instead make sure that your answer control picks catches this, the "custom error" will be displayed. Due to historic reasons, and the "." is used for other syntax elements, if you have a field of total 4 digits and 2 decimals, in the answer control you treat this as a 4 digit number.
So for example to limit e.g. 0.00 to 2.99 answer control should be 0:299, to limit 0.00 to 99.99 the answer control should be 0:9999.
So when using this, you have to make sure that the format, total digits and number of decimals, match your answer control, and the number of decimals you set for the sum.