This could be due to connection issue with the chrome cast device and is preventing your phone/desktop from finding your chrome cast.

Follow the below steps to fix this issue –

Android phone or tablets

Unplug your chrome cast device from power source for 15s and plug it back for chrome cast to reboot. Try casting again once rebooted.
Check if your chrome cast is on same wifi network as your android device. Kindly ensure they are on same wifi network and try casting again.

For android phone/tablet, turn on network by clicking on Settings> Google>Device & Sharing > Cast Options>. Turn on use network information and try casting again.