There is a current issue when searching for friends in-game on PC. This could be one of three different smaller issues.
It is worth knowing that partial name searches work and could be more reliable, so it's worth just trying a part of your friends name as a first try.
Search failed. Please try again
This seems to be happening when using the # in the search to find their full name/tag. The workaround is to just search the players name without any tag (for example just searching Strite instead of Strite#12ab)
Player not found
First, check the spelling and make sure you've got their name right!
If you're sure their name is correct there are two problems that can cause this:
Two byte characters such as those found in many Asian languages are currently causing problems. You can search one individual character at a time, but inputting more than one will cause a Player not found error.
There has been some confusion regarding crossplay; unfortunately, the game does not support crossplay between PC and Xbox. You won't be able to find friends who are on another platform.
However, there is crossplay between the different PC systems (Steam, Epic, Microsoft Store/Gamepass) - you should be able to find friends as long as you're all on one of those storefronts.
If you are still having problems after checking these, please submit a support ticket here: https://support.bossagames.com/hc/en-us/requests/new