You can not directly convert the timestamp to string because go lang does not support it. To convert it to string,
you need to first convert the timestamp to golang Time data structure and then convert it to string.
You can use the following to do this:
Ex: txntmsp,errN := stub.GetTxTimestamp() _ = errN time1 := time.Unix(txntmsp.Seconds, int64(txntmsp.Nanos)).String()
Note: you need to import the time package for this.