Stative verbs - verbs that represent states rather than actions - are rarely used with the continuous aspect.
The British Council has a page about this. Verbs of thinking and opinions (believe, doubt, guess, imagine, mean, remember, think), feelings and emotions (dislike, love, prefer. want, wish), senses (see, hear, taste, smell, feel) are given as examples:
She doesn’t know what to do NOT
She isn’t knowing what to do
It smells of smoke in here. NOT
It’s smelling of smoke in here
There are exceptions, of course (enjoy is often used in the continuous, and some of the others are used with continuous aspect either in certain circumstances or in colloquial usage).
For the modal verbs, the rule is absolute: they are never used in continuous forms. Indeed, they cannot be, because they lack participles. You can't say "I am woulding" or "I am shalling" or "I am maying" because those participles don't exist - and you can say "I am canning" only if you are using the unrelated non-modal verb "to can".
Modal verbs also lack infinitives. We often get around that by substituting a phrase. For example, we can't say "I will can", but we can say "I will be able to", and we can't say "I will must", but we can say "I will have to".
For the continuous aspect, we generally don't use substitutes: we don't say "I am being able to" (although "not being able to" works as a nonfinite expression in "Not being able to find your key is a problem"). "Be" is usually not used with the continuous aspect, although the possibility exists and is idiomatic in certain uses). On the other hand, "I am having to" is sometimes used as a continuous form of "I have to" (and therefore an equivalent for the missing continuous form of "I must").