When using bitcoin-cli I get an error regarding fallback fees when trying to send regtest Bitcoin. How do I resolve this?

bitcoind uses transactions it relays (and later see as mined) to estimate the feerate to set for transactions its wallet creates.

On regtest, you don’t relay then mine enough transactions so bitcoind‘s estimation buckets are not filled enough to give a reliable estimate. Without such an estimate the wallet, out of safety, prefer not to create any transaction unless you explicitly tell it what fee to use when fee estimation isn’t available.

This “fall back feerate” can be set up at bitcoind‘s startup with the fallbackfee startup option, expressed in BTC/kB.


         A fee rate (in BTC/kB) that will be used when fee estimation has
         insufficient data. 0 to entirely disable  the  fallbackfee  fea‐
         ture.  (default: 0.00)

Alternatively, crafting transactions on different nodes of your regtest network and getting them mined will result in estimation buckets to be filled.