In singleplayer Half-Life you do not get a HUD before you pick up an HEV suit, so place an
item_suit.
If you are making a multiplayer map and launching the game from Hammer (with "Run map" or the F9 button) or another level editor, the game is likely starting in singleplayer mode (even if you are mapping for a multiplayer-only mod such as Counter-Strike) so you need to either start a server from the game's menu ("Create Server" or "New Game" depending on the mod) or add
+deathmatch 1
to the launch options.