I've tried a few, and it looks like any 64 character hex number above 0 is accepted.
Any 64 character hex number between 1 and
n-1 will be accepted, with
n being the order of the secp256k1 curve which bitcoin uses.
How see mining transactions if i see no btc in btc explorer ? with electrum ?
If you see no transaction on a block explorer such as
https://btc.com, then importing to Electrum will make no difference as it will show the exact same zero balance. If there are no transactions on the address you have ended up with, then either you have no bitcoin or you have landed on the wrong address.