@Pablo888. It would not accept my documents and wanted a round trip ticket.  Are you entering by private car or motorcycle?
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  -@Enprize
@Enprize, I am entering by air. Although the e-visa allows application by land and sea too, I think that the problem is that a) you have to submit the travel document, b) you have to state what is your entry and exit dates, and c) you have to state how many days you plan to stay.
Obviously, the travel document should contain what would be your entry / exit date. The 2 additional questions are just verification checks. I suspect that the issue here is that if you don't have a return ticket, the authorities may think that you would stay past the expiration of the visa.
Another required document is a recent bank statement showing that you have sufficient funds for tourist activities. If you have this funding properly documented, I think that you can argue that you have a flexible exit date as you can fund an extended vacation. However this will probably need an in-person visit so that this can be explained.
BTW, my visit in February is to attend carnival and I also have all the hotel, sambadrome, and the usual tourist trap pre-paid documentation. I think that this type of documentation - although not asked - is evidence enough to show that I am indeed in Brazil on vacation - not to stay.
My suggestion here is to put yourself in the shoes of the Brazil authorities. Tourists are expected to return to their countries before the expiration of the visa. Hence, if you could gather all the documentation that can definitely prove that you will do this, I don't think that you would be denied the visa (unless there are extenuating circumstances).
Alternate way to prove land entry and exit - when I drive to Western Canada, I sometimes book a hotel near the border - because I need rest from the travel. The hotel reservation outside the country can act as evidence of exit from the country. For instance, I would book a night in a hotel in Blaine WA before entering Canada and then book a night after exiting Canada. The 2nd booking shows that I am following a schedule and intend to leave Canada. This would be my approach if I were to enter by land.
Bottomline, entering by land and sea is harder to justify than by air.Â
I suspect that you are being asked to provide a return ticket because the process would be easier to follow.Â
Can you buy a return ticket and then ask for a credit / refund later? I don't know how much the tickets from Panama to the nearest big city in Brazil is....
Good luck here.