Substitute function string issue

i wish to replace/substitute "[", "]" with " " in string received below

""param":[{"key1":"value1","key2":"value2"}, {"key3":"value3","key4":"value4"}]"

substitute(""param":[{"key1":"value1","key2":"value2"}, {"key3":"value3","key4":"value4"}]","["," ")

substitute(""param":[{"key1":"value1","key2":"value2"}, {"key3":"value3","key4":"value4"}]","]"," ")

I am receiving the below error

Expression evaluation error at function 'substitute': Signature length (4) does not match parameter length (12)

can someone help me understand the error here.

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