uniqueString is used to name some resources. The sample is "myUniqStr" which contains uppercase characters. This string is used to create the CFE S3 bucket (unless you specify a bucket name in "CFE S3 bucket"). If you leave the default "myUniqStr" (which you are encouraged not to) or some other value with uppercase and do NOT specify a bucket name, the template will fail stating the S3 bucket creation failed due to an uppercase character in the S3 bucket name.
If you do specify a bucket name in "CFE S3 bucket", if you use an uppercase character the template will fail with the same error.
The Unique String field will create a failure upon stack submission if it contains some illegal characters; such as hyphen
Suggested fix
change example Unique string without uppercase.
Add instructions in the template and on GitHub readme specifying the types of characters that are allowed in Unique String and "CFE S3 bucket" (if used).
Documentation link
https://github.com/F5Networks/f5-aws-cloudformation-v2/tree/main/examples/failover
Describe the problem
Suggested fix