Imperial Levels keep resetting on construction time

This response no longer applies after the 1.3.0 update

The rewards for imperial floors are calculated as follows:

((M1 * Q1) + (M2 * Q2) + (M3 * Q3) * 51)

Where M1 is the number of minutes it takes to stock an imperial floor and Q1 is the number of items stocked from that floor.

M2/3 and Q2/3 are for subsequent floors required to redeem the reward.

Basically, for every minute it takes to stock an Imperial floor item, you get 51 credits.

E.g., one of the missions is:
15 Rebel Secrets
1 Salvage
2 Battle Plans

The total reward is 13,515 and is calculated as follows:
Rebel Secrets = 10 minutes to stock
Salvage = 25 minutes to stock
Battle Plans = 45 minutes to stock

((10 * 15) + (1 * 25) + (2 * 45)) * 51 = 13,515

Tiny Death Star is still receiving updates occasionally, which add more levels - including Imperial levels. It's certainly possible that such a new Imperial level might enable you to complete this mission in the future.

It is also very much possible for the emperor to ask for a currently impossible level to be built. This could be considered an oversight or a bug - neither of these is confirmed and we can't really look into the developers' minds on this. Support for Tiny Death Star is also abysmal.

If you wish to proceed with the missions, you will either have to wait for an update that adds Imperial levels, or skip the (costly) mission.