How can we run regtest without transaction fees
This is a duplicate - see also  Does the Bitcoin Qt client have a minimum transaction fee?

Bitcoin-QT will protect you from sending transactions that are very unlikely to ever be included in the blockchain.  If you really want to try sending with 0 transaction fee, you will need to use another wallet software or patch and recompile the code.

It is highly recommended to include some transaction fees.  See estimates at https://bitcoinfees.earn.com/ (former "cointape") to figure out how much is needed.  Keep in mind that the estimates are very uncertain, they count it's like 90% probability that the transaction will go through within the estimated window - but then there is also a 10% probability that the estimate is off.  There have been times where transactions that should have gone through within hours have taken days or weeks to get through, though it's rare.

I often also consult https://jochen-hoenicke.de/queue/ to see what's going on, particularly the "mempool size in MB"-graph may be relevant.