Hello and welcome,
The employer is normally permitted to set the terms of employment, and may modify the terms of employment that are not in violation of a contract in place typically.
Since you have diabetes, which is normally recognized as a disability under the ADA, you may request that the employer provide you with a reasonable accommodation by not requiring you to obtain the class A license and continue to perform the job.
As long as the accommodation is not an undue hardship to the employer, they must normally provide the accommodation under the ADA.
Here is a link that summarizes what constitutes a reasonable accommodation under the ADA:
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