Hii @brother_p, check the version of your wallet.

New mandatory update of version 1.3.1.1
Multisend POW blocks / MN Payment Enforcement
Please update until 17jan

https://github.com/rodrigodepeder/nanucoin/releases
Linux Daemon - https://www.nanucoin.com/downloads/nanucoind-unix.zip
Linux Qt - https://www.nanucoin.com/downloads/nanucoin-qt-unix.zip
Windows Qt - https://www.nanucoin.com/downloads/nanucoin-qt.zip