kubectl returning cannot unmarshal string into Go value of type map[string]interface {}

If I run the command using echo, it seems to be a valid JSON

In fact, it does not. Look carefully at the first character of the output:

'{"data": {"FOOBAR": "value that I want"}}'

Your "JSON" string starts with a single quote, which is an invalid character. To get valid JSON, you would need to rewrite your command to look like this:

echo "{\"data\": {\"FOOBAR\": \"$FOOBAR\"}}"

And we can confirm that's valid JSON using something like the jq command:

$ echo "{\"data\": {\"FOOBAR\": \"$FOOBAR\"}}"   | jq .
{
  "data": {
    "FOOBAR": "value that i want"
  }
}

Making your patch command look like:

kubectl patch secret operator-secrets \
  --namespace kube-system \
  --context=cluster1 \
  --patch "{\"data\": {\"FOOBAR\": \"$FOOBAR\"}}"

But while that patch is now valid JSON, it's still going to fail with a new error:

The request is invalid: patch: Invalid value: "map[data:map[FOOBAR:value that i want]]": error decoding from json: illegal base64 data at input byte 5

The value of items in the data map must be base64 encoded values. You can either base64 encode the value yourself:

kubectl patch secret operator-secrets \
  --namespace kube-system \
  --context=cluster1 \
  --patch "{\"data\": {\"FOOBAR\": \"$(base64 <<<"$FOOBAR")\"}}"

Or use stringData instead:

kubectl patch secret operator-secrets \
  --namespace kube-system \
  --context=cluster1 \
  --patch "{\"stringData\": {\"FOOBAR\": \"$FOOBAR\"}}"

Found the error

kubectl patch secret operator-secrets --namespace kube-system --context=standard-cluster-1 --patch "{\"data\": {\"FOOBAR\": \"$FOOBAR\"}}"