How can I "Add more space if needed" for my 'reasonable error'? When I click the button no extra lines are added.

[peggygarden]

I wasn't sent my RMD and I don't have enough room to explain the error. The "Add more space" button doesn't work.

[Critter]

You don't need to write a novel ... cut the explanation down to brass tacks  or  attach a complete letter with a mailed in form. 

[fanfare]

When "explaining", IRS wants to know what you did. Include dates and amounts.

In some situations , it could be complicated. this is unusual.

[Critter]

Keep it short, simple and to the point.   The data input operator/processor  really doesn't care about a long drawn out story ... just tell them you made the mistake, fixed it and promise it will not happen again.  

For instance : 

Adding form 5329 for forgotten RMD distribution due to misunderstanding with the account custodian ... the error has been corrected and will not happen again in the future.

[fanfare]

Keep typing and see if some additional lines magically appear.