Much like everything in Animal Crossing: New Horizons, the Museum has to be constructed, unlike in other Animal Crossing games where it is already erected. In order to access the Museum and get Blathers the scholarly Owl to run it, you will have to go undertake a series of tasks. Not to worry though, we have listed everything you need to do below.
Firstly, after getting settled on the island, you will need to talk with Tom Nook in his tent and he will ask you if you could help him out. Donate 5 critters, either bug and/or fish (you can mix and match like 1 fish and 4 bugs, 2 fish and 3 bugs, etc).
Note: Whilst you can provide either fish or bugs, they must all be of different species. You can’t bring 5 of the same Tiger butterfly for example.
Upon donating 5 critters to Tom Nook, he will ask you to position a temporary structure for his friend Blathers somewhere on your island. It will take an actual real-life day for the erection of Blather’s tent. You can alter the time by going to the Nintendo Switch Date and time settings and disconnect Synchronise Clock via the Internet. You can change the time by a day.
With Blather’s tent erected, he will ask if you could continue to donate 15 more critters, be it bugs, fish and fossils (you will require a shovel to find fossils). Same as before, they must all be different species in order for them to register, you can’t give multiple specimens of the same species. Upon donating 15 more critters, you must wait for the Museum to be built.