Xiaomi Mi A1 won't show update to Oreo
I have the same device and this may help


There were two December updates for Nougat. You need to have 7.12.19 which is a prerequisite. My son also purchased the device a week after mine and he got the second update just yesterday and no Oreo yet, so I suspect that the roll out in batches is linked to the purchase date
Clearing Google services framework is not recommended but I went ahead and did it, since none of the users who did it reported any problems. Update didn't follow immediately but it came after about 7 hours (overnight)
Some users have successfully got updated after spoofing their location to other countries, using a VPN (US and Germany). I did this too but didn't work


Of course the fastest would be to visit local service centre and get it done free of cost