Two or more frames to animate means that your simulation time is very small or the probe setting you are using has very big step. This may be the reason you can check this by increasing the simulation time or if you are using any probe then reduce the time step.
I never got this error with HFSS but I am optimistic that by making one or both mentioned changes the problem will be resolved
I built a structure and used animation to E field.
It demands a sweep variable (let's say phase),
what is this phase belongs to? is it the phase in x-y plane as defined in polar coordinates?
If I am getting your question correctly just think in the terms of time as electric field is changing with the time so at any particular time it will have certain phase and certain magnitude. So if you will sweep it with respect to time you will get value of phase and magnitude at every swept point. Now just think about field only at particular phase the field will have a particular magnitude of electric field. This is the phase which you will give as sweep variable and resultant animation will give you corresponding electric field strength against that phase value.
Yes for sure try to define more sweep points but in that case simulation will take longer to complete as HFSS will analyze the model at all sweep points.
Good but for learning it is always better to follow some tutorial example step by step that will greatly help you to understand the working of HFSS.
It is better to have some proper example from theory and then try to solve it using HFSS and compare the results that will be the best way to learn HFSS. Without proper example you will not be able to learn and appreciate different functionality available in the software