eth-brownie: swapAVAXForExactTokens fail to perform a basic swap

Question

I wrote a simple python program whose aim is to swap the native token of the underlying chain, e.g., ETH, AVAX, for other tokens. In this case, my account holds 5+ AVAX and I am trying to do the following:

qty_out = 10*(10**mim_contract.decimals())      [10 MIM] 
path = [wavax_contract.address(), mim_contract.address()]
max_delay_sec = 1000int(time.time()+15)
avax_decimals = 18

And then I call this method

router_contract.swapAVAXForExactTokens(qty_out, path, user.address, max_delay_sec,
                                    {'from':user.address,'value':10**avax_decimals})

The program breaks, returning the following:

the method eth_sendTransaction is not available

I suspect this might be cause by a missing authorization to swap coins. Is there anyone who experienced something like this? If so, may I ask you for some guidance?

Thanks