It is a bit ambiguous but, given that the system would not allow you to complete the form 12 days before your planned arrival, the 7 day period is obviously a maximum rather than a minimum. The system will allow you to complete the form so long as the lead time is seven days or less. You can complete it six days in advance but not 8 days in advance. The reason for "fill in" rather than "fill ...
The form must be filled out within 10 days before the flight. Fill out the Entry Form within 10 days before your flight. What is the meaning of within in these sentences? Is it before 10 days before the flight? or Is it after 10 days before the flight? I.e. 8 days or 5 days or even 1 day before the flight is ok (even the same day as the flight)?
Is if you're treating the two days as a single length of time; are if you're treating them as multiple lengths of time.
Two days "is" or "are"? - English Language & Usage Stack Exchange
This same question was recently asked by you on English Language Learners wasn't it? I believe the answer there was that none of them are correct because all of them should say, "the Internet". Once that is fixed, then the only viable sentences are the ones that use "for the last few days", "in the last few days" and "in a few days". Although the meaning of the last one is different.
adverbs - The variations of in/for the last few days - English Language ...