Take a look at this thread, there are a solution for a similar issue (Re-Building the SD card image). Let's try that and let us know the outcome of it. Also, have you tried updating the packages like this other thread: Re: 'opkg install python-opencv' says, Unknown package 'python-opencv'.
I was able to install python-opencv and run my program by referring Re: 'opkg install python-opencv' says, Unknown package 'python-opencv'.
I shall try the other solution of rebuilding image and get back