If the 1841 does not recognise the CRYPTO command in config mode, then you do not have a version of the IOS with the features needed to configure a VPN. If you post the software version from the ‘<b>show ver</b>’ command, then we can confirm that.
Can’t advise much on SDM as I have never used it. I suspect you cannot copy it onto the router because it has not enough free flash memory. SDM also requires an IP address configured, the interface up (no shutdown) and the http server enabled (or maybe the https one).
Post some more specific information about the router and the IOS version, and possibly the config (remove any passwords, company information, and change IP addresses) and we can help you further.
<b>Not so sure, but maybe these will help:
ip http server
ip http access-class 23
ip http authentication local
ip http secure-server
access-list 23 permit 10.10.10.0 0.0.0.7 (replace with yours)</b>