Issue with Speck cipher on Arduino UNO
speckTxtKey[]= 0x65, 0x74, 0x69, 0x4c, 0x19, 0x18, 0x11, 0x10, 0x09, 0x08, 0x01, 0x00;


That's not how you assign an array.
And you never defined a type for the array.

It sould be:

byte speckTxtKey[] = {0x65, 0x74, 0x69, 0x4c, 0x19, 0x18, 0x11, 0x10, 0x09, 0x08, 0x01, 0x00};


EDIT:

I checked the source of the library to figure out how to use it, but it doesn't seem the library is meant for actual use.

SpeckTxtLen and SpeckKeyLen are macros inside SPECK_AVR.h, which are supposed to be changed there, not in your code.

The library also defines speckTxtKey inside SPECKenc_AVR.cpp with predefined test data based on the chosen length.

This library looks like a test implementation of the speck cipher, which was just dumped onto GitHub.