Some mobile devices use a feature called Dual SIM Dual Standby (DSDS). They allow both a physical sim and one or more eSIMs to be installed on the same device. The limitation is only one sim can be in use at a time, which means when you add a second SIM, you need to choose which SIM to use for data services. However it is smart enough to allow you to send and receive SMS on both SIMs; receive inbound calls on both SIMs (just not at the same time); and choose which SIM to use for your outbound calls.
This limitation also means that if you are using your primary SIM to make a phone call, you cannot download a second eSIM while the call is active. It can also mean that after you have downloaded your new eSIM, you may experience problems using the amaysim app, if you are trying to make a call on a different SIM at the same time.
So please ensure that you are not on a phone call when trying to download your amaysim SIM. It may even help to temporarily remove the physical SIM from the handset and use a wifi connection to download your eSIM.