RTA2 can't be issued as tenant doesn't meet minimum tenancy duration
If the previous tenancy is not recorded as a tenancy in MRI Housing Enterprise then the system tenancy duration will not include this previous tenancy. You have to have had a tenancy duration of at least 3 years before you an issue the RTA2 for your right to acquire application. You will need to add an associated tenancy to your RTA Application for the previous tenancy which is not recorded as a tenancy in MRI Housing Enterprise. I tested this on our internal system.
When I tried to issue RTA2 I got the same error as you. I then added an associated tenancy record to the RTA Application from RTA Application details > Purchasers tab > Selected the person > Tenancy History button.
Click add and fill in the additional tenancy details. This then included the persons associated tenancy in the tenancy duration. You can see that the system tenancy duration is showing 0 years 6 Months. But the Total Tenancy Duration is showing 4years 7months.
I could then issue my RTA2 without error as the total tenancy duration is more than 3 years.