Depending on the TLD, the registrar might be restricted from doing so. You should check what is the reason for the restriction with either the registrar or checking up on (g/cc)TLD restrictions
Domains have a life of maximum 10 years. Once you register a domain, you may renew it for 10 years only after it expires. This is so because until expiry 1 year is still not over.
Example: you register ispcircle.com for 2 years on 10-Jan-2000. The Expiry Date now is 10-Jan-2002. So you can renew your domain name for:
Before 10-Jan-2001: max Renewal allowed 8 years (as domain name is still 2 years old)
Between 10-Jan-2001 and 10-Jan-2002: max Renewal allowed 9 years (as last 1 year prior to expiry is still going on)
After 10-Jan-2002: max Renewal allowed 10 years (as domain has expired)
Note: The above example holds true for almost all domains with the exception of a few.
Renew it for as long as you can then renew it again later or move the domain to a company that may allow a longer renewal time.
Make sure the domain is worth the value to renew it that long to being with or your just throwing money away.