Try the following syntax:
SEARCH_PARAM=search(asm_mbr_user_parameter,"string")
IF SEARCH_PARAM>0
statement
ELSE
statement otherwise
ENDIF
The first line of the code, the assign operation, cannot be declared upfront somewhere in the top of the relations. This is important, You should define this kind of assign operation right before the if statement where you use the conditional with the parameter that comes from this assign operation, else you might get the same error you get above.
In report table relations, conditional operators such as <, >, ==, <> cannot be used along with system parameters such as asm_mbr_name, asm_mbr_type, etc. You have to first make a copy of those by assigning them to another param.
Make sure you only work with one system parameter such as asm_mbr_name at a time, and then close it with ENDIF before working with another. IF statements can be stacked one into another, but I doubt stacked IFs will ever work with conditional operators as said above.