Go to your output apk files (Usually \build\app\outputs\flutter-apk).

Use keytools to get the SHA1 value (run in terminal / git bash):

keytool -printcert -jarfile app-debug.apk

Convert the HEX value to base64 to get value that ends with =. (i.e. Use this site. Be aware to have at Input type selected Hex. And delete all : between SHA1 key groups -> AB:CD:EF = wrong, but ABCDEF = correct)

Update that value to key hashes under your android platform.