You're trying to pass a string to a float:
Syntax: native TF2_AddCondition(client, TFCond:condition, Float:duration);
Usage: client - Player's index.
condition - Integer identifier of condition to apply.
duration - Duration of condition (does not apply to all conditions).
Simply use StringToFloat()
PHP Code:
decl String:arg1[65], String:duration[255];
GetCmdArg(1, arg1, sizeof(arg1));
GetCmdArg(2, duration, sizeof(duration));
{
TF2_AddCondition(client, TFCond:TFCond_Ubercharged, StringToFloat(duration));
}