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
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.