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CAMT (corporate alternative minimum tax) calculated on Form 4626 usually applies to very large corporations with average AFSI (annual financial statement income) exceeding $1 billion dollars. According to Form 4626 instructions, unless a filing exclusion applies, a corporation must file Form 4626 to determine whether it is an applicable corporation and, if it is classified as an applicable corporation, to calculate CAMT. However, one of the filing exclusions for Form 4626 is that it is not an applicable corporation under the simplified method.  

For corporations with AFSI (Adjusted Financial Statement Income) of less than 500 million, users can confirm that they meet the requirements of the safe harbor method (simplified method) directly on the 1120 and do not need to file Form 4626. If the wrong box is checked on Line 29c of the 1120, the IRS will send a notice stating you need to include Form 4626. Please see the screenshot below for additional clarification. 

Since you have received an IRS letter regarding this issue, you have two options. You can either file an amended tax return with an updated Form 1120 with line 29c box checked yes or you may also respond to IRS letter according to the instructions in the letter.