You will not be able to set the summary method to Formula for the time dimension if using a time-based function like previous() If you try you will get an error like the one below:
However, depending on what you want to show you may be able to use the other summary methods to achieve this. For example if you wanted to show what the value was in month 12 of the prior year you could set the summary to Opening Balance ( month 1 of current year = month 12 of prior using Previous).
Break your formula into multiple line items and you should be able to get your results. Add Two line items Previous Premium and Previous GTV - Both should have SUM at Summary Level and then do the simple division in the Output line item and change the summary to Formula.
@Akhtar.shahbaz unfortunately it doesn't work that way - simple refer will still not have the right values at aggregated levels because technically there is no formula, it is just refer.
@Adisulyadi97 Do mark the 'Accept reply as solution' to a reply if it resolved your query! It encourages community members in general and also helps future members who may have the same question but may skip over the thread assuming it has no solution