Final answer:
The error is in the phrase 'more happier' which should be 'happier', and 'has married' which should be 'had married'.
Step-by-step explanation:
The error in the sentence "She couldn't have been more happier on her wedding day, even if she has married in Hawaii." is in the underlined portion containing the comparative adjective 'happier'. When using the phrase 'couldn't have been more', the adjective should be in its positive form, not comparative or superlative. Therefore, the correct phrase should be 'couldn't have been happier' without the word 'more'. Additionally, the verb 'has married' is in the wrong tense for this conditional context and should be 'had married' to correctly form the third conditional, indicating a hypothetical situation in the past.