@madam There's no rich text control in Report Designer. The text box is the only text element, and its ConvertHtmlToText property strips HTML rather than rendering it. You don't need rich text for this, though. A text box's Value can be an expression that mixes your fixed wording with data fields, so the paragraph rewrites itself on every invoice. The stock invoice form already does this for the billing address block and the email body.
Since your invoices come from a schedule, the generated invoice's date is set by the schedule run. Build the paragraph off that date and the dates change themselves each month:
='This invoice covers services for ' + Format('{0:MMMM yyyy}', [ARInvoice.DocDate]) + '.{br}{br}Your fixed wording continues here.'Format takes any .NET date pattern, so '{0:d}' gives a short date and '{0:dd MMMM yyyy}' spells it out. If you bill in arrears, wrap the date in DateAdd([ARInvoice.DocDate], 'm', -1) and it shows the prior month. {br} gives you a line break, and set CanGrow to True on the text box so the paragraph wraps.
One limit: a text box has one font and style. If part of the paragraph needs to be bold, put that part in a second text box above or beside it.