This is one instance where the error message and solution in Excel is explicit.
We are only allowed a certain number of different cell formats (depending on what version you are using).
A cell format includes anything on the list that we get with menu Format/Cells. eg. font name, font size, number format, borders ..etc. A single cell with a single difference can use up one of our allowance. So you have a reformatting job .... Edit/Copy then Paste Formats wherever possible. Even empty cells contain some sort of formatting.
For example, if numerous fonts (names) are being used you could select the whole sheet and change everything to a single font. That would reduce the number of variances straight away. Similar with font sizes.
Although it may not be entirely relevant here, take the time to select all the "empty" rows at the bottom of the sheet and delete them. Same with columns to the right. If they have been used earlier for something Excel will "remember". This often reduces file size considerably. They could contain some odd formats, which will be converted to the default.